In reply to as646: as646: If you're looking to train for the Royal Marines and you don't do much running now then forget about the bleep test and put it to the back of your mind. Buy this book - Royal Marines Fitness: Physical Training Manual.
Focus your training around avoiding injury and increasing your running base. By all means do intervals (or better still hill reps) and throw one longer run in a week but if you jump in to three interval sessions and a long run you will greatly increase your chance of injury. It's not to say it can't be done but don't go from doing little to doing loads straight away. Just put down the miles (preferably off road) and strengthen your legs with squats, split squats, heel lifts etc.
What times are you getting for your split 3 mile run? I take it you've passed your pjft? If I was you I'd be more concerned about getting my return 1.5m run down to below 10 mins.
I think there's a potential royal marines forums, you might get better information from that.