Available as a direct purchase for iPhone and iPad.

The PCGamingWiki recommends "ThirteenAG's Widescreen Fix" for modern resolutions, which is typically downloaded as a .zip file and extracted into the game directory. Downloads - Rockstar Games

Search for Grand Theft Auto III or use the package name com.rockstar.gta3 . It is a paid application that requires approximately 1GB of free space.