As I have previously written, 3-defender systems have an odd place in contemporary football, often considered obsolete, they nonetheless have the tendency to pop up again and see success with certain teams.. The key to understanding 3-at the back systems is that they are actually 5-defender systems, as in all but the rarest of cases the system utilizes wingbacks, who are evolved from fullbacks and are best described as wide midfielders with responsibility to cover the defensive wings - as compared to most wide midfielders who are usually primarily attacking players. The left and right defenders of the trio at the back play deeper and narrower than fullbacks, and thus the formation is generally described as having 3 center backs.
There are those who claim that these formations are more tactically interesting, but I actually beg to differ, and this is admittedly because, as I see it, teams looking to use a 3-defender formation are more often than not looking to play more defensively, and want to use three center backs to provide additional cover. It also helps to understand in almost all cases, 4-defender systems are a misnomer as well, and the nomenclature can be quite counterintuitive.
Teams that are said to employ "4 across the back" almost always employ a deep-lying central defensive midfielder, what I like to call the "ACB" midfielder (Auxiliary Center Back) and it can be said that these are 5-defender systems as well. Additionally, fullbacks play higher than center backs, and have important roles to play in helping provide width to the attack and make overlapping runs with midfielders. On the whole, both back-3 and back-4 formations essentially use 5 defensive players, with either 3 or 2 center backs, who are the dedicated "out and out" defenders in the system.
It actually goes beyond that, with regards to the flexibility and adaptability of "back-4" systems. More often than not, they are designed to always keep 3 defenders back, and in a back-4 system that 3rd man can be the CDM or either fullback, depending on the game-play situation. At the end of the day, I feel like the tactical nuances of the back-4 are overlooked and underrated, and I would consider a more accurate description of the approach to be the "'W' defense".
The key to the "W" defense in many cases is the CDM; I don't feel like it is a stretch to say the CDM on many teams is the most important man in the pitch. In the case of the Sounders, that man is Osvaldo Alonso, and after the 2010 season both his fellow players, and the Supporters, felt as though he was the team's most valuable player.
Last year we saw Sigi settle on a system that was best described as having two defensive midfielders; once Alonso settled back in after injury he and Nathan Sturgis played together in the preponderance of the Sounders competitive matches from mid-July onward. This season, from all indicators, we can expect to see a return to what we saw a lot of in season one, with a "stacked" central midfield of Alonso and more of a box-to-box or link-up CM in front of him. In this case, we can expect the Sounders to run the "W" defense. In this regard, 2011 may very much be Ozzie Alonso's year.
Looking elsewhere in the "W", once again a lot will be asked of James Riley, as steady a presence in the Sounders lineup as anyone else; he has in fact been a stalwart at the right back position. I would go so far as to say his absence from the lineup for any considerable length of time due to injury would constitute a crisis.
Center back will see three experienced, proven MLS players. Jhon Kennedy Hurtado was, along with Alonso, the linchpin to the Inaugural year defense that co-lead the League with Houston for fewest goals allowed, and for his trouble he was one of three nominees for defender of the year in '09. When he was lost to injury in May of last year, the position was thrown into a bit of a crisis, with Tyrone Marshall suddenly appearing not up to the task, despite a solid Inaugural year in his own right. Patrick Ianni was employed as a CDM for a time - which gives reason to believe he can and will be used there again. Jeff Parke, thrown into action after a long layoff, took a while to get his feet under him but once he did, and his pairing with Ianni took hold, it turned out to be a solid combo.
It was the left back position which was relatively unsettled in that Inaugural year, and the position was a bit of a revolving door - even featuring Nathan Sturgis at times - until Leo Gonzalez arrived mid-season, and provided an attacking spark that wasn't necessarily represented on the score-sheet. Last season he could be seen to put himself into some panic positions - never good for a defender - and on the whole wasn't as solid a defender as his opposite fullback, Riley. There is an expectation that his spot will be challenged, with Tetteh providing a more marauding, "wingback"-style alternative - a useful tool to have in the kit.
Additional depth comes from Wahl, who can fill in at CB and LB, and Scott, who will primarily be the backup RB. Taylor Graham is still around and will primarily be a reserve. Roger Levesque also looks to be an occasional possibility at right back.
I should probably pause to point out what I consider an important point in the nomenclature I am using. I prefer to reserve the term "wingback" for the hybridized fullback/winger position that has sprung up in 3-defender systems. I would rather not use it to describe particularly aggressive fullbacks in back-4 systems, keeping in mind that pushing high up the pitch into the attack and generally being a marauding presence on the wings is all in the job description of a fullback.
There are extremes. Barca has demonstrated a hybrid back 3/4 system, predicated upon the willingness of their fullbacks Alves and Maxwell to go forward and the CDM Busquets to drop back as an ACB. This is where the "W" defense comes from, at least contemporarily. With Busquets as almost a "sweeper" forming a back three with Pique and Puyol, Alves and Maxwell could be considered wingbacks. With Busquets as a CDM, and Alves and Maxwell in a more defensive position - we would see a more usual back 4 with the CDM as a 5th defender of sorts. In all, the average position would be a "W".
The shape of the midfield has a big effect on fullback positioning. In a diamond, like we have seen at Chelsea, the average position of the fullbacks is actually higher than the CDM. This is because the diamond lacks wide midfield roles and it is incumbent on the fullbacks to push high to provide width to the attack. This is why we see the marauding runs of the likes of Bosingwa and Cole with Mikel playing as an ACB.
In the Sounders case, it may not be quite this extreme, but we do know that high pressure from the wingers - or outside mids - has been a key to the Sounders attack, and this - along with the more withdrawn role of Fredy Montero - has meant that the formation may be closer to 4-2-3-1 than 4-4-2 (particularly with the two CDM's used last year). These high wingers mean that there is space in behind, and the fullbacks are the most likely choice to fill that space.
Alonso will certainly be free to roam, however, and is best left unleashed to provide cover where he needs to. The descriptions I am using are really about average positions, as compared to nominal starting positions. They are what you would see on a heat map, as compared to a lineup sheet. It is probably best to remember that it is somewhat of an abstraction to talk about defining shapes in soccer, but for the sake of calling it something, I call it the "W" defense.