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.