Source: Superman vol.1 #354-5, 361, 364, 368, 372 (1980-82)
Type: Alternate future
Turns out though that with three Supermen defending New Metropolis, there isn't much to do for each of them. Superman-III keeps the Purists in check, teams up with his dad and grandad, and saves Daily Planet editor Jimmy Olsen's grandkids (Clark and Lois) from crooks, but since he doesn't sleep, he has a lot of time on his hands.
So whatever happened to Superman 2020? He became Superman 2021, then his back-ups disappeared from the pages of Superman, and as far as we know, his alternate future was gobbled up by the Crisis. And he unlived happily never after.
I'm not going to make you wait a whole week or more for Supermen I and II. We visit 2020 again this Monday and Tuesday! And my friend Snell wrote a series of posts on Slay, Monstrobot about the strip last year, and as is usual for him, they're comic masterpieces. Check 'em out!