The PSP port of Grand Theft Auto III is a faithful adaptation of the original game, with some notable compromises to accommodate the PSP's hardware limitations. While it may not be the most visually impressive version of the game, it retains the core gameplay and charm that made GTA 3 a classic. Fans of the series and portable gaming enthusiasts may still enjoy this iteration, but it may not be the best representation of the GTA 3 experience.
: Converting PS2-era textures and models into formats the PSP’s Media Engine could process without crashing. gta 3 psp port
All 73 original story missions and side content are fully playable. The PSP port of Grand Theft Auto III
To understand why an official GTA 3 PSP port does not exist, you have to look at the PSP’s guts versus the original PC/PS2 hardware. : Converting PS2-era textures and models into formats