Trevor would get his head kicked in. Don't get me wrong, Trevor is a complete mad man who could take out most GTA protags, but Niko is a hardened Serbian war veteran who saw his friends killed, I doubt he'd think twice about battering Trevor to the ground.
As for CJ vs Franklin, CJ would definitely win. Franklin's a bit more physically imposing, but he's also a bit of a bitch. CJ on the other hand was an OG who brought the Grove Street to the top in the height of the crack epidemic, Franklin doesn't have the same level of experience.
Are you kidding?
CJ was the very definition of a bitch.
All he did was whine, all he did was follow orders.
Hell, the only reason he even went back to the Grove was cause he couldn't say no to Sweet being a stubborn asshole.