From 0:00-3:00, perform 3 rounds of:
3 pull-ups
6 push-ups
9 squats
From 3:00-6:00, perform 3 rounds of:
4 pull-ups
8 push-ups
12 squats
From 6:00-9:00, perform 3 rounds of:
5 pull-ups
10 push-ups
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.