Really mate you need to be a lot lot closer to 20 mins. On your PJFT you have to run 1.5 in under 10 and they are looking for closer to 9 than 10. Therefore your 3 miler shouldnt be much over double that, ie, 20 mins
With regards to improving it, do lots of small sprints at absolute maximum effort with small breaks in between, and chuck in a few runs over 3 miles, 5 - 6. If your body is used to running more than 3 miles, you will find 3 miles a lot more comfortable, then if you work on your sprints at maximum effort you will increase your lung capacity and recovery time so be able to put more effort into your longer runs with out so much suffering!