I would say Trevor is most likely the more durable than the other two. He is the least afraid of getting hit and being ready for the impact will drastically increase your ability to take punches. His willingness to get hit also means he'd be more aggressive because he won't be afraid to take a punch in order to give one. I'd also say that he most likely has the most punching power.
I kind of agree, kind of disagree.
In normal gameplay, Trevor actually doesn't have much pain tolerance, at least no more than Michael or Franklin. He reacts just like they do to being punched in the face or the stomach, and groans, "Ugh, give me a break!" when hit by a vehicle. He also doesn't always seem as eager to get into fights as you might expect; if you walk into a store during a robbery, he'll put his hands up and stay down until they're gone just like the other two.
As for his stats, while Trevor has the highest default strength of the three, it's entirely possible for Michael to be stronger than him when they first reunite if you've been playing enough tennis beforehand*. It'd be really interesting to see how a fight between the trio would go if their stats were all maxed out and, say, Trevor was as good a driver as Franklin.
It seems like the only sure thing Trevor has going for him in a fistfight is his special ability, which, to be fair, is a huge advantage. But he can still be ragdolled by explosions or vehicles, and Michael could conceivably slow down time to dodge his punches.
*Interestingly enough, Michael gets into more fistfights than Trevor during cutscenes, and against tougher opponents.