Hal Higdon has some great free programs, especially for relative beginners.
Your 3 miles, 3 times a week is exactly what he recommends a person be capable of to begin his Novice Half Marathon Program. And, it's a 12-week program, so this is like serendipity for you on the timing.
Pay attention to the suggestions below the schedule. It's easy to just focus on the schedule, but he gives a ton of wisdom that will keep you out of trouble if you listen to him.
Here's the schedule:
http://www.halhigdon.com/halfmarathon/novice.htm
Good luck! And, when in doubt be conservative and take a rest day. Better to be a little undertrained and healthy than overtrained in injured for your first race.