I am not a running expert but I think it is totally doable. Just build from your three mile base, a mile more per week or 5 min more on an out and back course should get you an extra mile every week. So in two weeks you should be running 5 miles. Also remember your brick work, that first run off the bike is always a killer.
I have always heard that if you can get a wet suit you should use a wet suit. They keep you more bouyant so you swim faster.
I'm really excited for you a 1/2 IM is a big accomplishment.
Jones