Grand Theft Auto 3 Walkthrough:
Patriot Checkpoint Missions

The patriot checkpoint missions aren't marked, but you just get into the patriot and try to hit all the checkpoints without the time running to zero. Each checkpoint you hit gives you more time.

There is one at the Supa Save in Portland, and one in the picnic area by the dam on the Vale island.

Grand Theft Auto 3 Walkthrough