I think any 3rd party characters should be a truely iconic videogame characters,
therefore the third [and final?] space should go to one of...
Representing:
Hudson - Bomberman
Capcom - Megaman
Leon would also be a good choice for Capcom, representing the Resi series and an alternative/rival to Snake.
I would list Namco - Pac-Man,
but I can't see a complete move set for the character, but if there are any third party assist trophies then Pac-Man, Dig Dug etc... might be good to represent Namco!