In your GTA3 folder, run MTAServer.exe, then go into the launcher as normal and click "Start Multi Theft Auto" and start, when in credits alt+tab out, click "Send Game Request" then alt+tab back to game, in a few seconds your game will start.
You may get an error that says "Another Client Connected From The Same IP Address" in that case its ****ed and theres nothing you can do about it, either join another server or play Single Player GTA.
I advise you buy Vice City and play that, it works much better and the game is more enjoyable than GTA3 is.