On the 3 mile run, aim to complete the run back in under 10 mins. The pass/fail criteria on the run-back is 10 mins 30 secs, but if you can achieve it under 10 mins, you should feel fairly confident with that element when attending PRMC for real.
This will probably be your first true test of the RM Fitness Assessment (RMFA) carried out in relatively rapid succession & should bring home a true appreciation of where you are now & where you need to be in order to pass PRMC & the need to layer your exercises.
The way to view it is as a free dummy run for your benefit & to enjoy it, don't be worried about it. Ask all the questions you want as the guys running it are the same RM's than run your PRMC in Lympstone, so you can get a true appreciation of what to expect.