yall talk about switching formations like playing video games .. This is barca we are talking about, 4-3-3 is not a formation , it's the identity of the club, Young players are nurtured since the very beginning to play the diamond.. the only question is how to play it .. small détails, like : are we playing a classical 9 or a false 9 , are we playing classical wingers , forwards as wingers , or even midfielders in the wings .. that is all .. Last time i remember a Barca coach experimenting the 3-4-3 was Rickard in the anfield against Liverpool in 2007 ( no left backs ), the result was not only flattering , it was hilarious , if you don't believe me go Watch the highlights on youtube , it could have been and should have been 3 or 4 nil by halftime , It was THAT bad for barca . The 3-4-3 is a set up conceived for defensive tems , it' about playing 3 center backs , closing down spaces and playing on the counters, who plays the system as of 2016 ? Juventus , and Italy .. do they play any type of offensive football ? No . the talk of Playing Messi in midfield and all those Pro soccer formations would never happen because they are unrealistic , sure Messi can be pushed back to midfield , but in this case the number of forwards would automatically go from 3 to 2 and messi some sort of a free offensive player even if he sat deep. and this is already happening right now he's more of a midfielder.
back to the original idea .. No , Barca will always play with fullback and 2 center backs .. 4-3-3 is what you will always see with some minor modifications , 3-4-3 that you will never see
You've missed an important part of this club history if you think 4-3-3 is the only formation that has given us success.
Anyway, formation itself is really not that important, it's what type of players are playing and what are their roles that define whether you play offensive or defensive. For example, 4-2-3-1 is the most used formation nowadays but you can play it with Pep's tiki-taka style (in Bayern, where he had the real "9" in Lewa) or parking the bus (Mourinho) way. On the paper both Pep and Mou play the same formation, on the pitch there is a big difference. One is playing with 60-70% possesion, the other one with around 30-40% but both are feeling confident with the way their teams are playing. Most people consider one style as offensive and other as defensive, although there are people who says that Pep's tiki-taka is also defensive. And they are not totally wrong. Pep himself has admited that keeping possesion is the best defense for him as the opponents can't score if they don't have the ball.
3-4-3 in their original form is actually too offensive for todays football. Cruyff said 25 or so years ago that he prefers 5-4 win over 1-0 win. And high results were very normal in his days. Nowadays more teams prefers to win 1-0. Sadly, but true. Euro was the best proof of that. Most spectators considered tournament dissapointing because there was not enough action and goals but if you ask coaches most of them will say football was tactically well executed and that they learned a lot. It's just the way toward football is developing right now. 5-7 years ago everyone was trying to copy tiki-taka because we were successfull. Most of the teams couldn't really work it that way because they didn't have as much talent (Xaviesta+Messi) as we've had. So they started to look for systems that would neutralize tiki-taka and eventually they've succeded. That's why Lucho has gone some steps away from pure tiki-taka to again make us successfull. But again, he couldn't have done that without as much talent as we have and other teams don't have MSN so they will just continue to play their way. Even Pep has started to adapt his philosophy in Bayern and he'll have to do it even more now that he'll coach in a more competitive league.
You mentioned Italy and Juventus. They play 3-5-2 which is turned to 5-3-2 when defending as both wingbacks (Liechtsteiner-Evra in Juve and Florenzi-Darmian in Italy) pull back. That said, I acutally think Italy has played some solid football in that formation in Euro, especially considering the fact they were without their best midfielder (Verratti) and without good strikers. It will be also interesting to see how Alves will work in Juve system. But again, this Italy/Juve 3-5-2 system that you consider defensive has absolutely nothing to do with 3-4-3 played in Cruyff''s time and if we'll ever play 3-4-3 again it will be a lot more similar to Cruyff's formation than Juve.
Look for another example: Sampaoli has played 3-5-2 with Chile (we'll see if he'll stick with that now in Sevilla) and you can't say they've been playing defensive. So as I said above, it's more about players and their roles than just formations.
Our current team is the best suited for 4-3-3, there is no doubt about that. But we have enough talented and versatile players to play other formations like 4-2-3-1 (as was mentioned above we actually played that formation in some games last season and we could play it again as Rakitić is well suited to play DM alongside Busquets; and we could also play Masche or maybe Samper there), 4-4-2 (with diamond in the midfield and Messi as »10«) or 3-4-3. We won't play any of that formations longterm because we'd have to make some pretty big changes in our squad to make that happen (especially have enough depth for playing those systems) but we can play in different formations in some games.
Adapting formation to players is good and what smart coaches do when neccessary (longterm injuries or suspensions) or if they think they can play better that way against certain opponent(s). What is killing coaches is when they try to adapt players to fit them into their favourite formation. That usually ends with players playing way out of their position/comfort zone.
I think if we play 3-4-3 or better said 3-1-2-1-3 with MAtS; Masche-Pique-Umtiti/Mathieu; Busquets; Rakitic-Iniesta; Messi; Arda/Denis/Rafinha-Suarez-Neymar or something similar (we could also just play 4 midfielders with Iniesta probably the most advanced of them and MSN in attack) in some games it would work as all of the players that would play »out of positions« in this case (Messi as »10« and whoever of the mentioned would play RW) are all used to play that position. Of course the team should train that formation for some time before playing it in serious games.