What PvP arena formats are available?

Both formats are duels you opt into. Fights are set up through PvP invites sent to another player, so there is an invite step between you and any arena match rather than combat being forced on you. That also means finding opponents is a social act. You need someone to accept.

The split between the two sizes is the real choice. The 1-on-1 format puts everything on what you personally bring to the fight. The 3-on-3 format is the larger team version of the same arena combat, and it is the only other size on offer. There is no middle option and no larger bracket above it.

Arena duels are one branch of Aniimo's multiplayer rather than the whole of it. The game also runs online cooperative play, including challenges you can take on alongside other players. If you want to play with people instead of against them, that side exists separately and does not route through the arena.

Not settled: What the three in 3-on-3 counts is not settled: it may be three players per side, or one player fielding a team of three Aniimo.