I would suggest using those first 10 weeks to get used to training each discipline 3 times per week. As other people have suggested, focus on your swim and getting a private coach is a great idea. Over the next 10 weeks your swim training should consist of almost purely drills and form work. Don't worry about intensity at all. Since swimming is the weak link and you have ready access to a pool you may want to even try to swim up to 5 times in a week, even if you only go into the pool to spash around from one end to the other for 15 min.

Long rides won't hurt you at all. They really help to build your overall endurance base. So go for it on the weekend, but during the week, you'll need to do speed work on the bike.

Here's a suggested plan:

Week 1: 2 swim days, 2 run days and 1 or 2 bike days
Week 2: 3 swim, 2 run and 1 to 2 bikes
Week 3: 3 swim, 3 runs and 2 to 3 bikes
Week 4: rest a little, 1 to 2 swims, 2 runs and 1 to 2 bikes
Week 5: Repeat week 2
Week 6: repeat week 3
Week 7: 3 to 4 swims, 3 runs, 3 bikes
Week 8: repeat rest week
Week 9: Repeat week 6/3
Week 10: repeat week 7 or 5 swims, 3 runs and 3 bikes.

All of the workouts above should be relatively low intensity and medium to longer duration. You're building an aerobic base so you should never feel like you're going hard for a long time. Having said that, it is helpful to through in short (30 to 60 sec) bouts of speed play in yor bikes and runs. I wouldn't do it for your swim, due to lack of experience.

That should help a bit.

Take the rest of your time to recover because the plan you go on will be designed to start with you rested.