How come when you unlocked all three islands in gta 3 you still cant get six stars?

If you have the original copy of the game, or if you have a virtual copy from a legitimate source, this should not happen. As soon as you are able to cross over to Staunton Island, you will be able to get six cop badges in wanted-level, if you keep shooting police/swat/FBI guys and/or destroys their cars and the chopper/choppers. It is a simple matter of quantity. Keep shooting the police defense forces (usually blowing choppers and higher cop-leveled cars (SWAT or FBI cars/military trucks yields a faster cop aggression).

If this still does not work - there is most likely a mission you have yet to complete in order for you to be able to get that last cop badge.