Abs, legs, then arms. If you push the pace on each movement, just remember that you'll get 2 chances to rest that respective muscle group by switching to another. What that means is: identify what movements need to be pushed, and which ones need to be paced. Fast sit-ups are not that different from slow sit-ups so you don't need to challenge that too much. It's the runs and snatches that can be tested. See what happens if you try and keep a quicker pace than is comfortable each round, and then recover on the sit-ups. Abs, legs, then arms. It's only 15 minutes but see what you can get.