There have been rumors that Xavi is keen on a 3-4-3 formation next season, although nothing has been confirmed. FC Barcelona have played all their matches in preseason in a 4-3-3 formation, so it seems the likeliest thing is that that's how they'll play at least to start La Liga.
Still, it's hard not to think Barcelona don't see the 3-4-3 as an alternative formation, perhaps for specific games. Thomas Tuchel's Chelsea switched between 4-3-3 and 3-4-3 last season, and so perhaps it's not surprising that Xavi and Tuchel have competed for similar players this summer transfer window.