Grand Theft Auto V will have three different protagonists. Grand Theft Auto IV had three different protagonists, too, when you include Johnny Klebitz from The Lost and Damned and Luis Fernando Lopez from The Ballad of Gay Tony. Some modders have figured out a way to switch among them and Niko Bellic, the way GTA V will with its characters.
It's a work in progress, so it has some problems still. But with the script installed, players can switch between Niko, Luis and Johnny using hotkeys. Once you leave a character, the old one will stroll around or attempt to flee the cops if he has his wanted level. You can see how the switch is handled in the video -- the view zooms out, flies over to the new character's location and zooms in, picking him up mid-action.
Each character has his own health, armour, ammunition and vehicle. "Those are saved each time you change [characters] and will be loaded when the game starts." You can see more in that video above. To download the script, visit the link below.