Extract the zip/rar file on your desktop and place the .exe file(the trainer) in you Gta 3 folder, For the teleporter to work you have to first startup the game, then minimize it and open the teleporter option. Thee should be some preset locations along with the trainer, Check the folder were you extracted it and load those files. Now select the place you want to teleport to and maximize your game.
I may be wrong, Its been a while since I've played Gta3.