Again, speculation runs both ways.
It could "JUST" lock down which of the starting companions you keep and which ones you don't, but I seriously doubt that at the end of Act1 there will be an arbitrary lock which means no other potential companions worth meeting in the later acts!
Sorry, that seems far fetched.
Companions die, your decisions might cause current companions to leave... I do not see us being "stuck" with our choice of 3 companions for the rest of the game.
I could be wrong, but until more info / or the game itself comes out, we both simply don't know.
