Not enough information to help you out here. Specs?
If your pc didn't meet minumum BF3 specs and upgrading your pc isn't an option, then owning BF3 really does you no good. BFBC2 is a very good game, and quite honestly plays similar to BF3. IMO, the rush maps are actually better in BC2 so there is lots of fun to be had in that game.
Idk BF3 at low settings ran at 20 fps on my PC while it runs at about 45 FPS on BFBC2, so playable