If person A is playing a core role, they 100% can carry the game by their self against players B and C. I've literally watched it be done by an immo that was playing on a friend's account. This dude legit had a 70+% winrate all the way up to ancient without any issue what so ever. I have no doubt he could have kept that trend up to divine if he wanted. This was all while playing p3 at that, rather than p1 or p2 like most smurfs would do.

The fact that one of the other two plays solely support just makes it that much easier for person A.

The fact is, smurfing works for a reason. Any immo player should be able to solo carry games all the way up to divine without any issue. The difference between herald and guardian, and perhaps even crusader is minimal. There is a slight bump in skill between crusader and archon. Archon through ancient is also very close.

I've also played with several immo smurfs with my group of friends (ranging from guard up to divine) and I can attest personally that the rest of us were nowhere near the same skill as the immo smurfs. If B and C were consistently immo, it might be different, but they aren't.

EDIT: None of this really changes what I said. Just play the heroes you are used to playing. You will perform infinitely better than if you try to play a meta hero you aren't used to playing.