As I mentioned in my "Formations" post many weeks ago, the 3-5-2 is a complex species with many breeds, and every time it is deemed "extinct" it has the habit of popping back up again. What is notable is that 3-defender systems are usually used to provide a spare man at the back for teams that want to bottle up their opponents and defensively stifle the game.
So-called "three at the back" formations are in fact the embodiment of the first lesson in formations: Look beyond the labels. There is a serious nomenclatural flaw in how we think of formations currently, and as Jonathan Wilson points out [skip to the last section last section] there will probably be a time that people find our current system for recording formations to be insufficient.
3-5-2 from 4-4-2...?
It seems to make sense that the idea of a 3-5-2 is to pull a defender and replace him with a midfielder to shift the balance of a team more to the attack. This is probably because we see this late in matches when a team is in need of goals (or we see a center back pulled for a forward). It turns out that this isn't really how the 3-5-2 works. To understand what's going on we need to take a closer look at the wide players.
In the 4-4-2 wide midfielders are generally more attacking players. This is to say that they are going to be primarily focused on attack, with obvious defensive responsibilities as well. [there are exceptions, but in the interests of keeping this to a reasonable length, I'm overlooking them for now] In a 3-5-2 the wide midfielders are primarily focused on defense, with obvious attacking responsibilities. This position has come to be called "wingback". This is because in the 3-5-2 the wide midfielders are actually evolved from fullbacks.
The Fullback.
Before I go on, I need to re-iterate an important point: "Fullback" specifically refers to wide defensive players. A center back is not a fullback, and a fullback is not a center back (although there are numerous players who can play both, they are different positions entirely). It turns out that fullbacks are very important to contemporary football. It's always trite to make generalizations, but you are going to find that if you want to field a good soccer-football team, you are going to need good fullbacks.
If someone were to call me and ask "how do I play great attacking soccer" I would tell him/her to find a pair of center backs who can communicate almost telepathically, a defensive midfielder who can destroy whatever is beautiful, and a pair of fullbacks who are the fittest players on the team.
If you are playing 4 at the back, and are using wide midfielders or wingers, you have two layers of wing players. Go ahead and play those wingers inverted, and the fullbacks will cover wide areas on the attack. Wingers don't run the touchline and boom crosses any more, now that's a fullback's job.
In a 3-5-2 the wingbacks ARE the wide players in the formation. With your fullbacks essentially pushed higher into the midfield, you're going to want a third center back to help cover.
Is a "back three" really all center backs?
Yes and no. Obviously, with a back three it makes no sense to keep all three players narrow at all times, but it could also certainly be said that none of the three would be what could be truly called a fullback. The left sided and right sided of the three are generally going to play narrower and deeper than a fullback. It is also worth noting that the central of the three is traditionally a "sweeper".
That's right, we're going to look at what Barca does again!
It's not just Barca, [read from the beginning this time] but as I pointed out with the articles linked at the beginning of this post, the "sweeper" seems to be returning as a CDM - a variant I like to call the ACB, or "auxiliary center back" . What we see from Barca (amongst others) is a hybrid back 3/4. it could be said that the defense is split into two layers: a "back two" consisting of the two center backs, and then a layer of three consisting of the fullbacks and CDM; all forming a "W" shaped 5-man defense.
This is where we reveal the truth behind the "back 4". The purpose of the back 4 is to always allow for at least three defenders to be present. If your right back pushes, the left back tucks in to form a back three with the two CB's. If both fullbacks push, the CDM/ACB falls back as a "sweeper" and the two CB's drift slightly wider. This is the epiphany that the fluidity of contemporary attacking football has spread to the defense.
And then there's Chile...
I'll get to Chile in a bit, but first I have to explain that the predominant species of the 3-5-2 the last decade was the 3-4-1-2. The 3-4-1-2 never really left South America, and had a pretty glorious few years with some European teams in the early 00's. Italy used it to great effect in the '00 Euros, and it certainly had it's heyday in the 2002 World Cup, where is suddenly seemed the de-rigeur shape.
The 3-4-1-2 is what you think it is. One of the two CDM's will probably actually be a deep-lying playmaker, but that still means you have a back three supported by wingbacks and an ACB. It isn't all doom and gloom, fortunately, if you don't want it to be. Brazil certainly provided plenty of entertainment with it in '02, but for the most part this is for teams that, as mentioned at the beginning, would prefer to more or less pack it in. Going back to that fictional phone call about attacking soccer, I would certainly insist that less "defenders" does not equal more "attackers".
So there MUST be an example of a team that actually ran THREE defenders? It turns out there is. It was Chile, in this last World Cup cycle, under Argentinean Marcelo Bielsa. What Chile decided was that essentially their wingbacks didn't need to be wingbacks. It amounted to a 3-diamond-3 with the rear player in the diamond being an ACB tasked with helping out the back three, and an "enganche" supporting a front three with true wingers.
There is an important caveat, however. International football lags far behind club football in terms of tactics, and as such "quirkiness" - for lack of a better word - has a place there. What this means is that the limited ability of national teams to refine tactics and personnel, compared to the club level, means that we will see things "work" that may not necessarily be successful over the grind of a league season.
So what of a "true" back three?
What I would call a "true" back three - a left back, center back, and right back not supported by wingbacks - actually happens relatively often as a reactionary tactic if a team is down late and looking for a goal. Sub out a center back for an attacking player, and you're there. There seems no real reason to sacrifice wide attacking players just because you'd rather have three out-and-out defenders. It seems only a matter of time before some manager somewhere decides he'd like to start out a match with an aggressive lineup and makes the leap.
It will have the effect of stifling your fullbacks just a bit, but that may not be a big factor if you don't have great fullbacks to begin with. I would say that if you have great fullbacks, you should let them play fullback, and not inhibit them by making them play in a back three system.
But maybe you don't care. If you are going to have essentially 5 defensive players it may make more sense to you to have them arranged as a 3-2 rather and a 2-3...
example of a 3-2 defensive formation:
-------X-------
-X----X----X-
-------X-------
----X----X----
--X---X---X--
a more "standard" 2-3 alignment (showing the CDM as what I call an ACB):
-------X-------
-X----X----X-
-------X-------
-X----X----X-
-----X--X-----
Both these arguments put fullbacks and CDM's on the same layer, which isn't far from the truth. In both the diagrams above, I am showing only 5 layers instead of 7, which is essentially melding the attacking mid and 2nd striker layers up top and the defensive mid and fullback layers at the bottom. If I wanted to go ahead and show these shapes in 7 layers, i would do it like this:
-------X-------
-------X-------
-X----------X-
-------X-------
----X----X----
--X---X---X--
In this we have essentially replaced the wingbacks with wingers, and maintained what would still be called a 3-5-2. It is likely that at any given moment one of the two outside backs will have freedom to push forward, still allowing for what amounts to two center backs, and your two CDM's to flow where needed to provide the proper coverage.
The following should be a familiar shape to Sounders fans:
-------X-------
-------X-------
-X----------X-
-------X-------
-------X-------
-X----------X-
------X-X-----
this is the 4-4-2 evolved to include a withdrawn striker, attacking wide midfielders, and a CDM. It is very close to the 4-2-3-1 which has widely replaced the 4-4-2 as the "default" formation in contemporary, global soccer-football. This formation provides greater freedom to your wing defenders, which is what you'd want if you have fullbacks capable of pushing forward and contributing to the attack.
As for the Sounders...
There is no need to re-invent the wheel, essentially. If you want more attacking players on the pitch, and only three out-and-out defenders at any given moment of open play, this is already built in to the "back-4" strategy. Marauding fullbacks are a common aspect of the 4-4-2 family of formations, and if you want to use a player like Tetteh as a left back then it fits right in without having to change the rest of your shape. Proper balance will be maintained with subtle positional shifts and individual players' understanding of how their roles may shift slightly with personnel changes, rather than entirely changing the shape of your side.
While the common tactical argument is that 3-5-2 is done in by the changing shape of attacking and the move to only one out-and-out striker, I would argue that it is also done in by less inherent flexibility. This is a bit counter-intuitive since the original inspiration for 3-5-2 was to provide for more midfielders which could play in a supporting role both attacking and defending. I suppose the difference is that most formations are now actually referred to as having 4 bands, and this is actually particularly true for 3-5-2, for which the simplistic description of a band of 5 midfielders was never entirely adequate. Just like "4-4-2" can mean a large variety of quite different approaches, "3-5-2" can mean 3-3-2-2, 3-4-1-2, or 3-2-3-2, to name just a few.
While we are comfortable with splitting the former 1 band of midfield into 2 bands, and in same cases splitting the former 1 band of forwards into two bands, we haven't been willing to split defenses into more than one band. If we were it would be helpful, as "4-4-2" may reveal itself to not only be 4-2-3-1, 4-4-1-1, or 4-2-2-2 but maybe even 2-2-2-3-1, or 2-3-3-1-1, or even 2-2-2-2-2! In this case, we may reveal the truth that a back 3 really adds a man on the deepest layer, instead of taking one away.