I spent over 10 years in the Marine Corps. We run 3miles for our PFT. My best 3miles was 18m47sec. Later in my career i was running on average a 20-21min 3mile (age catches up with us all).
The best way to increase your time or performance is to run further then your target goal.
If you have to run 1.5miles for score. Run 2.5-3 miles for practice. If you have to run 3miles for score as we did, run 4-5 miles during practice.
The way that prepares you for when you actually run for score is this:
Since you are used to running further you will have more "wind" or endurance. Do not blow that right out of the gate, when you take off the line.. Just find your natural pace. Do this for HALF your distance.. Natural Pace
Once you are past your halfway mark, step it up slightly (you still have wind/energy to burn as you are now used to going much longer then you will need to).
When you reach the last 1/4 of the run, put EVERYTHING you have left into it until you reach the finish line. This is dang near a sprint and makes up a great deal of time for you.
As you become more fit and increase endurance you can push harder sooner, BUT never push hard first then back off, because your body will want to naturally back off more then you think you have, it will hurt your score in the end.
Some additional Notes***
Unless the test is going to be on a treadmill (even then I would not suggest training on it, because they are not going to give you a treadmill to operate from when you go into combat/war zone)or conditions are such outside that running outside is impossible, DO NOT TRAIN FOR PFT ON A TREADMILL.. Running on a mill is 2x easier than running normally. This will give you a false sense of time/distance.
Get GOOD running shoes and develop good running habits to ease the strain on your knees.
Mix it up, if possible swim 1.5-2 miles once a week rather then running. This will seriously build your endurance while also giving your joints a much needed rest. Swimming will also strength the "little" muscles you do not use much in not only your legs but the rest of you body, these muscles are generally used to stabilize joints and as such, help prevent injury.
It is amazing how much you will improve your score, because normally people put too much into the first half and fall behind on the second half. It does not work that way well; the way I explained above is how I used to get my Marines into better shape and better scores.
