He chose a 4-3-3 system, although he likes 3-4-3 as well
Videogames company EA Sports published a video starring Leo Messi. The presenter invites the Argentine ace to talk about football systems. First, he suggests a 4-4-2 that does not suit Messi's taste, who claims he prefers a 4-3-3.
After that, the presenter asks him what he thinks about a 3-4-3, and Messi replies: "Let's start with a 4-3-3 and we'll see about it later."
You can see Messi's appearance in the video from the 20th minute.