That's the spirit! It looks like a reasonably standard sprint distance and looks like a fun-orientated event so good for a first go.
I guess the swimming is very dependent on where you are now and how much you want to put into it. It's very technique orientated which is the reason most people find it to be the hardest discipline - although luckily for most it's also the shortest in the race. In the worst case scenario you could get through it realatively easily by doing breaststroke and you still won't be near the back of the field (most of the slower times on the previous results will be doing breaststroke). If you want to perform a bit better though you'll need to learn freestyle. How difficult that's going to be will depend on where you are right now. Can you swim front crawl (freestyle) at all?
Because of the technique needed I think lessons would be the ideal way of learning. A coach will pick up straight away on your major flaws and get you a passable stroke pretty quickly imo. Other than that there are plenty of vids on youtube, and some good books around. The one I bought is
SwimSmooth which is really good, although it doesn't look readily available in the US. The most important things are breathing and body positioning. Poor breathing limits your oxygen supply so getting that sorted is priority (imagine holding your hand over your mouth for 4 seconds in every 5 while on the bike and you can picture how any flaws in your technique will have you gasping).
The good news is that despite all the stories I've heard about people trying it out and barely managing one 25m length of the pool without drowning/swallowing tons of water, all these stories seem to result in the people picking it up really quickly.
I got pretty lucky with the swimming. I used to swim in a club when I was younger, and even though I gave it up when I was about 11 (so 20 years ago) I think the technique had been stored up somewhere in my head and I picked it up again pretty quickly. Cos of this I'm not 100% sure what it's like to go into it completely green.
As for times, I think that if you learn to maintain a steady crawl stroke then 500m probably shouldn't take more than say 12.00. The elite can do it in <7.00, my current time is prob about 9.00, and I'd estimate that I could probably breaststroke it in about 14.00. That said, you can probably add a little onto those times for it being in open water - choppy water and jostling for space with other swimmers can slow you down.
As for running I think that, as an already fit guy, you could easily get to 3 miles within 6 weeks, but you won't be overly comfortable with it in that short space of time. The sooner you incorporate running into your training the better, even if you start really easy, like doing a couple of half-mile (or less) runs each week.
How about this for an idea. You could start tagging a very short run onto the end of your rides once or twice a week. This way you don't have to give over additional time to scheduling a separate session. You're already warmed up and are already wearing sports gear. When you get home, just stick on your running shoes and just set off on a really short run, even if it's just a few minutes round the block or something, and you don't even need to run the whole way. You could walk 2 mins, run 2 mins then walk 2 mins back home. Build that up slowly and you'll have a decent running base in no time at all, with the advantage that you're used to running off the bike.
Going back to times. If we make a very conservative estimate I think we could put the swim at 15.00, the bike at 50.00 (slow to save some for the run) and the run at 30.00, then we get 1:35 which is not too far above average. With more than 6 months to go before the event, with some well-structured (note 'well-structured' doesn't equal 'ball-breaking') you can crush that estimate imo.
Edit: Holy crap, just hit post and saw what a huge wall of text I'd just written!