How do you get to the other Islands in grand theft auto III?

Answer 1

Play the game! By doing missions with Luigi, Joey's and other shady thug hirers, you can unlock the other islands through the bridge, the subway, and the tunnel. Also, for all the game cheaters out there:

Cheat code: Right, Black, B, Right Trigger, White, Down, Left Trigger, Right Trigger. This will enable your car to briefly fly when you fly over a ramp. Go to where the broken bridge leading to Staunton Island is and enter this cheat. The fly up the bridge and you can soar over the massive gap to the new island.