My guess would be a cross between 4-3-3 and 3-4-3 used on numerous ocasions by Pep and Tito(last time during the famous 4-0 thrashing of milan), its a basic 4-3-3 with the right winger playing very centrally and the right back playing an extremally attacking role to provide width on this side, on the other side the winger stays wide without the ball stretching the backline even further. The left back plays a more defensive role and the backline shifts to the right when in possesion providing cover for the adventurous right back. This formation creates space for Messi since the centrebacks cant really close him down with Suarez up front and the central mids will find themselves 3v4 against 4 extremally good ball players. Suarez is also one of the best players in starting and organizing the pressing high up the pitch.
Barcelona 4-0 Milan: Villa plays centrally and allows Messi space between the lines | Zonal Marking
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This is how I lined up and did it in FM:
View attachment 346571
We won 3:0 match against FCP, and Messi did a hattrick