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.
Raphinha, Ousmane Dembélé, Robert Lewandowski, and Jules Koundé have all been linked to joining Chelsea, to some level or another, before joining Barcelona.
In addition, Andreas Christensen has already moved from London to Catalonia, and César Azpilicueta and Marcos Alonso are rumored to be headed that way.
Typically, a team will keep four senior centerbacks to cover two positions in a four-man line. Last season, for various reasons, Xavi kept six: Gerard Piqué, Ronald Araújo, Eric García, Clément Lenglet, Óscar Mingueza, and Samuel Umtiti. However, Umtiti was barely used (making only one appearance) and Mingueza was frequently deployed as a right-back. In fact, looking at the squad after Xavi arrived in the middle of the season, it was clear he counted on three main centerbacks: Piqué, Araújo, and García. Lenglet was the fourth option, and Mingueza was essentially a third-choice right-back. For all intents and purposes, Xavi had four centerbacks, really.
Today, Lenglet and Mingueza have been let go as the coach does not count on them anymore. Umtiti will probably not play much if at all, if he doesn't leave at some point.
The main three from last season are still here (Piqué, Araújo, and García), but there are two additional ones: Christensen and Koundé. Sure, Piqué is getting up there in age but he would probably be counted on to be the 4th choice centerback, if nothing else. Araújo and García are young and talented. Christensen and Koundé are surely here to play. Signing two centerbacks and having five (well, and Umtiti) seems like overkill.
Enter the three man line. A team that plays with three at the back necessarily needs more centerback depth, since it deploys one more every game. Last season, Chelsea used five centerbacks with some regularity: Antonio Rüdiger. Christensen, Thiago Silva, Trevoh Chalobah, and Azpilicueta, the latter also as a right-back. They also had Malang Sarr make a handful of appearances. Basically, they had more senior centerbacks since they played with a back three.
If Barcelona played with a back three, then having five centerbacks seems pretty reasonable. Azpilicueta could also provide additional depth in the position.
Xavi used the 3-4-3 formation at Al-Sadd with some regularity. At Barcelona, he could line up with Jordi Alba as a more defensive winger on one side, and someone like Raphinha as the more attacking winger on the other. Pedri could play as the most advanced midfielder, with Gavi and Franck Kessié as box-to-box midfielders, all supported by Sergio Busquets in the holding role.
Alonso, a more attacking left-back by nature, could serve as Alba’s deputy.
There have also been links with bringing in Iñigo Martínez from Athletic Club. He’s been linked with Barcelona many times throughout his career, but it’s never happened. It would surely be overkill to sign him this window, but if Piqué retires soon, you could see why Xavi would be keen on bringing in a left-footed centerback - again with a three-man line in mind.
The other possibility is that Xavi simply wants one of his defenders in a four-man line to stay back a bit more. With Koundé or Araújo or Azpilicueta as a right-back, and Alba or Alonso on the other, he would certainly get that. Meanwhile, rumors say Sergiño Dest - a more attacking right-back - could be on his way out.
The thing about this is that it would be pretty unusual to sign someone like Koundé to moonlight as a right-back. And if the idea is that one fullback stays back, and creates a de facto three man line... we haven’t seen that in preseason, really.
But, it’s still early days. Preseason might not fully reveal all that Xavi is cooking. It’d be unusual for him to plan to use a 3-4-3 as his go-to scheme and never try it out in preseason, but perhaps he only thinks of it as an alternative for the moment.