The ESRB have recently re-rated GTA III and GTA Vice City as PS2 Classics for the Playstation 3, indicating that a PSN release for two of Rockstar's finest is almost certainly on the cards. San Andreas was notable by its absence, and the PS2 Classics download service is still yet to make the jump to Europe.
Mind you, if you've got an early PS3 model, backwards-compatibility with discs is still an option.
GTA III recently released in its entirety on iOS and Android. [Siliconera]