Ohhhh boy! Triathlon is fun.
Here are my comments:
1. A general fitness base is good. Swim, if you're not already! At least a few times before your first tri, you'll want to do what is called a "brick" workout, where you ride your bike first, and then go for a short run straight-away. This will also give you time to set up a little mini transition area in your driveway (or wherever) and practice that, too.
2. Bike fitness translates so much to the other two disciplines, especially the run. The more consistently and well you can ride your bike, the better suited you'll be for the run portion, as your legs will be less fatigued than if you only ever ride so many as 12 miles. You should aim for more rides in the 25-30mi distance range.
3. Are these pool or open water swims? If the latter, practice an open water swim in a wetsuit at least once before your race, if possible. Wetsuits are awkward and they can really throw some people off, even if you're a decent swimmer. Also, if they are open water, practice bi-lateral breathing in the pool, as well as sighting. There are a lot of links to youtube things for techniques on that (sighting) if that's totally foreign to you.
As for the bike... honestly, I would borrow a mountain bike from someone for the trail triathlon, if possible, and invest your money in a more road-specific bike. While you *can* ride a mountain bike (or bike with suspension) on the road, it is far less efficient. Plus, for training, you'll want a more comfortable ride for the road than what a mountain bike would be able to do for you.
I could go on an on about the bike stuff, but that would be my initial comment. If that doesn't work for whatever reason, PM me/come back here and I'd be happy to check your local craigslist links and make suggestions.