How to play gta3 on iphone 3gs 6gb?

I have iphone 3gs that haves 6gb of memory and i installed gta 3 but its too laggy.
My iphone now haves 2gb free memory
What should i do

Unfortunately, problems with lag don't have to do with available memory space but the hardware itself. In other words, the phone model is the problem. It is old, outdated, and the game was not meant to be played on the 3GS. Sorry.