Barcelona has regularly utilized a 3-4-3 formation this season for various reasons. It allows them to get more of their best players -- central midfielders -- onto the pitch. It's also a better fit than a 4-3-3 for Javier Mascherano, who isn't exactly a physically imposing figure in a traditional back four. When Carles Puyol has been hurt or playing fullback out of necessity, Mascherano has stepped into defense, and he's a much better outside central defender in a back three than he is a traditional central defender in a back four.
Whenever Barcelona have played any team that uses two strikers or teams they're much better than who play any formation, they've gone to the 3-4-3. With the team's best natural left back Eric Abidal out for the season (and probably done with his career), playing with a back three makes even more sense for Barcelona now than it did earlier in the season. However, it's not a good matchup against Chelsea.
Whoever starts on the wings for Chelsea is going to do a lot of cutting in from wide positions, which is likely to pull a back three badly out of shape. There will either be large gaps in the penalty area or lots of space for the fullbacks to overlap and cross if Chelsea's wide forwards make the right runs against a back three. If Chelsea can find wide-open lanes for Ashley Cole to cross the ball into the head of Didier Drogba, it's probably their best avenue to scoring goals against the Blaugranas.
When Andre Villas-Boas was in charge, Chelsea failed to capitalize on their matchup advantages against Napoli's back three in the first leg of their Round of 16 tie. They were much better in the second leg of that tie and made Napoli's defense look terrible. If Barcelona opts to go with a back three, Chelsea's wide players can't get caught doing nothing but marking Barcelona's attacking wide players, which is what happened in the first leg against Napoli. It's a big matchup advantage, and they will need to take advantage of it.
Even though Barcelona would like to avoid playing Puyol as a traditional left back in order to put him in a natural position and bring another central midfielder into the team, the potential drawbacks are too great. They're going to dominate possession with a 4-3-3 anyway, and Guardiola should probably stick with it.
We'll have continuous pre-game updates and live game coverage in our Chelsea vs. Barcelona, UEFA Champions League Semifinals StoryStream. For more on the two teams, head over to Chelsea FC blog We Ain't Got No History and FC Barcelona blog Barca Blaugranes.