According to cube.ign.com, publisher Rockstar Games denies that they are working on GTA3 fr the GameCube.

About a month ago, we (and several other sites) reported that there were very strong rumours that GTA3 would come to GameCube. Website cube.ign.com contacted a representative at Rockstart Games. He told them that Rockstar is currently only working on GTA3 for the PC and X-Box, and that there are no plans for a GameCube version at this time.

Damned, one of the coolest games ver is not coming to GameCube.

Source: Cube.ign.com