use a walkthru
http://www.robsonherlock.clara.net/ps2/gta3walk.htm
i beat 'em all, but i dont remember how though. i moved on to vice city.
i remember the mission about gettin the gang cars.
1) there's a mafia sentinel parked near the other place (the place on the street from the car dealership, u did missions for the guy, then u killed him).
and take the long way out.
go in thru the tunnels and come out near the car delalership.
avoid the chinese people and the mafia. avoid mafia at all costs. they WILL shoot u w/ a powerful weap and kill u.