If you have GTA III on your iPhone 5, you'll notice that it has the iPhone 4/iPod Touch 4G graphics (no shadows, lighting, low framerate, ect.). This gta3.set (taken from the PC version) file should give you the enhanced iPhone 4S graphics, as well as drastically improving the framerate. The draw distance might not be perfect, but whatever.
You will need either iExplorer or iFunBox.
1: Scroll down until you find a folder named "com.rockstargames.gta3ios".
2: Go into the "documents" folder
3: Replace the old gta3.set with the new one
1: Click the "iFunBox Classic" boarder.
2: Click the "User Applications" box.
3: Click the "gta3" box
4: Go into the "documents" folder and replace the gta3.set