Not bad at all.
Thompson hit 211 last year; with 178 through 16 games, he's on pace to shatter that. So if you average out to 200 per year, let's give him a generous 15 seasons compared to Ray Allen's 17-and-counting. That puts him at 3,000 made three-pointers at age 36, giving him the all-time record.
Obviously, this is preposterously early and there are a million things that could change. But it does show you the ridiculous pace Thompson is on and how special of a shooting talent he is.