Polygon recently got the chance to interview Rockstar's VP of Creative Dan Houser, who talked in detail about making a game with three protagonists and switching between them. It's well worth reading the whole article, but he's a couple of excerpts we thought were interesting:
 

"Any moment you could go and see what they're up to and it relates to what point you're up to in the real story. You just get a real feeling for them as existing outside of you controlling them, so that just kind of added this great ambient storytelling quality and fun to it.
 
"You jump across and Trevor's doing something crazy. Michael's stuck in some kind of existential hell hole of his own making, and Franklin is getting into his trouble."

"You'll miss lots if you don't pop over to [a character] from time to time," Houser said. "You'll miss lots of the stuff they are potentially getting up to, but you won't miss key plot points. It will be little details about what their lives are like, based on what you've done in the plot. You'll miss those obviously if you move the plot past where those are no longer relevant, and just the sheer number of things you can see them get up to, you'll miss them if you don't keep going to have a look for them."

"Everybody thinks they're the star of their own movie at some level, don't they, in existence? So to each character, they are very much the star of their experience. We really wanted to focus on each of them having their own sense of progression, their own adventure, but where they're also so interwoven that it doesn't break apart in the same way as say, the episodes from GTA 4 plus the two mission packs breaks apart into three very clear stories with small points of crossover.
 
"In this, they're much more tightly interwoven to the point where they almost become the same thing, but, to any one of them, at any one moment, they should always feel like they're not someone else's supporting character; it's their life, it's their adventure."

 
To top that off, a new very high resolution artwork has come to light featuring Chop.