From 0:00-3:00, perform 3 rounds of:
3 pull-ups, 6 pushups, 9 squats
From 3:00-6:00, perform 3 rounds of:
4 pull-ups, 8 pushups, 12 squats
From 6:00-9:00, perform 3 rounds of:
5 pull-ups, 10 pushups, 15 squats
From 9:00-12:00, perform 3 rounds of:
6 pull-ups, 12 push-ups, 18 squats
Etc.
*Continue for as long as possible until you cannot complete 3 rounds within the 3-minute window. Every 3 minutes add 1 additional pull-up, 2 additional push-ups and 3 additional squats.