According to this video, Capcom originally intended for Code Veronica to be the third numbered entry of the series, while RE3 would remain a spinoff (i.e. a tie-in to RE2). But pressure to capitalize on RE2 meant they had to reverse that and release an RE3 sooner, so it became Nemesis as that was quicker to develop.
I'm not sure how accurate this is, but it seems like it would make sense.
I'm not sure how accurate this is, but it seems like it would make sense.