Little Nightmares 3 adds cooperative online play for the first time in the series, but it does not support crossplay between competing platforms. The development team says they are “still considering adding it,” in an official Q&A, but at launch, there is no crossplay. That means PlayStation and Xbox players cannot join each other, and PC and console players are also separate.
There is limited cross-generation support inside console families, so PlayStation 4 and PlayStation 5 users can play together, and Xbox One and Xbox Series X|S users can join one another. The reporting also indicates Nintendo Switch 1 and Switch 2 players will not have cross-generation support at launch.
Local co-op is not available, so online play is the only option for teaming up. For convenience, the game offers an friend pass, which lets a second player download a free copy to join a game when the owner has purchased it. The purchaser will be the account that earns trophies or achievements and saves progress. Likewise, the game does not support cross-progression across competing platforms, though you can transfer cloud saves when moving between generations in the same console family.