GTA III Tips and Tricks: How to access the locked Islands early on the game

Like other GTA's, it is possible to access the locked areas prior unlocking them officially in GTA 3. To do it, you must get a vehicle and drive to the top of the Porter Tunnel. Then park the vehicle at the middle and at edge of it with the left door facing it. Then get out of car and you will phase through the water land on the roof of the tunnel below the sea. Now you must follow the tunnel without falling out of the top. To see where you going, bring down the camera. Eventually you will find a invisible all, at there go left and fall. You will land on the road somewhere near the Rockford Hospital. With that, you will be on Staunton Island. To go to Shoreside Vale, just pick a car and take the Porter Tunnel. To return to Portland, take a boat docked on Newport and get out on at the little pier on Callahan Point.
Having access to Staunton and Shoreside earlier allows you to get hidden packages and complete side missions. Unlike on LCS where some things couldn't be done, in this game pretty much everything can be done. The only things you cant do is finish the Emergence Vehicles Export and Shoreside Vale Export. Rhino, FBI Car, Barracks OL (Only on PS2), and BF Injection. Aside from that, everything else can be done.
Going to Shoreside Vale early is also important to know if you had the Purple Nines in your Save, as there's a infamous glitch related to it. Once you passed that mission Rumble, the Purple Nines will be gone, and if you start a new save from there, the Purple Nines won't spawn back. That makes the mission Uzi Money impossible to be completed, thus preventing 100% completion to be obtained. To avoid the glitch, you must remove all the save files and start a save from fresh booting the game. On PS2, you can remove the Memory Card, while on PC and Mobile you can move away the saves to other paste. Once you started the fresh save and saw the Purple Nines there, you must save and always load from there when wanting to start over.