Also worth noting I don't think there was any hard date set for the films during their release.
Aliens gives us the year '79, meaning
Alien takes place in '22, but it being specifically 2122/2179 wasn't part of the plan originally. It was intentionally vague.
So they might have meant
Alien3 took place in the 23rd century when they wrote it.
@SM correct me if I'm wrong