RB3D is the patched version of RBII, but it was also sold as its own game. A bit odd, I know, but at the time most online connections were dial-up, so large downloads weren't possible.
Despite owning all three RB games (RB, RBII and RB3D), I still prefer the original. For some reason, it just felt like a better game -- or at least more entertaining -- than its sequels.
Anyway... here's what RB3D offered:
1. GLIDE support, which means it'll only produce high quality graphics (for 1998) if you have a Voodoo card. Modern video cards can't take advantage of this without a GLIDE wrapper.
2. Numerous bug fixes and an improved flight model.
3. Improved multiplayer capabilities, allowing for a player total of 76 (as opposed to 10 in RBII) in a game.
"If winning a debate is based on whoever gets in the last word rather than on who best articulated and defended their point of view, then I guess you won. Congratulations."