<Erik> (this is in reply to comment #5, which you've written seven years ago)
Play like you think that your opponent doesn't cheat - I don't think that it's good advice.
Sometimes when you realize that you're playing a cheater, of course frontal accusation will do no good as they will get angry or ignore you, but some subtle changes in the way you play can do wonders...
This is something that works against those cheaters who don't know their business and play rather slow, as they aren't sophisticated enough to use technology well.
When you've spotted one of these "slow-moving cheaters" you can start shifting the game towards positions where a computer won't be able to win in few moves - avoid obvious continuations, favor keeping the position closed, give checks if they slow him down, go for meaningless maneouvering where he can't exploit it directly and immediately, and above anything else, play faster than he does... you can flag such a cheater in many cases. Then of course, never play the guy again and report him to chess.com as soon as the game ends. Even if you won.
It's still possible to cheat in 3 minute games. Don't ask me how - I really have no idea. But I've seen it done, and I've seen people banned.