I am running 3 miles soon; what can I do to increase my chances of doing it in under 18 minutes??

Jogging daily. Timing yourself every jog. Break the run into small pieces then practice sprinting through. Rinse, repeat ad infinitum and you'd see improvements.

it 3 kilometres not 3 miles...thats easy stuff though...first of all u have to train every day mehn,,secondly u have to train everyday and thirdly,u must train everyday i repeat...to be force man no be beans brother.....Start with lets say walking on the first day,then by day ,,2 short step short distance jog,day three short step long distance jog,day four long step short distance jog and day five long step long distance jog..keep up the momentum,make sure u check ur blood pressure before embarking to make sure theres no issue...finally the best trick in jogging is that u shud eat light within d period,eat more fruit and water..thats gives u endurance and stamina...also do press up and leg exercise to resuce tension.goodluck