Learn to breathe correctly. If you do not know how to breathe correctly during the course, then you won't be able to run it, or run with a large amount of energy.

Breathe in every two steps and exhale on two steps. Initially, it will be difficult to adjust if you never did, but then the breath will have you on the machine.
Strengthen the heart. A strong heart is necessary for the development of endurance. To develop it, you need to seriously do a cardio workout.
For such training developed a lot of different exercise equipment such as exercise bike, elliptical trainer, rowing machine. Although you do not need to enroll in a gym or buy expensive fitness equipment, if your goal is to strengthen hearts, to run a specific distance in a specific time.
Bike you can substitute Cycling, elliptical, Jogging on skis, a rowing machine can be replaced with a boat or kayak.

Also to train the heart muscle fit running, intense Hiking, long swim at the pool or in the pond.
Stop and go to work or school transportation. Hop on your bike or go by foot if distances allow. Do not use elevators, walk the stairs on foot. Of course, if you don't live and work on the hundredth floor.
The trained heart will help you not to turn sour in the middle distance and to maintain a given pace for all three miles.
Don't undermine yourself to the respiratory system. In order to run 3 km in 12 minutes, your average speed should be about 15 km/h. now think about it, if you can maintain that speed throughout the race, if you smoke. So don't complicate your task by extraneous factors.
Divide the distance into 3 equal segments for 4 minutes. The first hundred metres you need to run fast enough, rest 4 minutes, run easy, not much bothering the body. The next 4 minutes you need to run moderately fast, but running was not a burden, otherwise forces are run is not enough. The remaining time will have to work and run at the expense of character. In the last hundred meters is still to strengthen his will and to finish with the maximum rate at which you are capable.