I reckon if Rockstar was to give out the GTA III Era GTA games for free, like GTA 1 and GTA 2, GTA III would most likely be the first one to be released for free. San Andreas is a long shot, as it is one of the primary money gainers for Rockstar Games, due to its popularity, whereas GTA III is not so popular, as it lack features from GTA Vice City and GTA San Andreas.