In my limited experience, finishing (time irrelevant) a marathon on 35-40 miles a week (peak), 3 runs a week is probably doable. I can't imagine getting away with only 2 runs a week, and I'd feel much more comfortable on at least 4 runs a week.
Realistically, you'll want to get in at least one 20-mile run before the marathon. Doing something like 10 miles Wednesday, 5 miles Friday and 20 miles on Sunday represents the "easiest" maximum mileage week I can think of. Fitting cycling and swimming around that should not be impossible.
Coming up with a traithlon/crosstraining system that gets you enough aerobic/cardio fitness to get to the finish line should be doable, especially if you're doing 100 mile weeks on the bike already. What you'll need is to strengthen up your legs to handle the pounding and abuse the marathon will feed them.
I haven't combined the two disciplines yet, but I'm looking to run a marathon next spring, before the triathlon season starts up. I'll probably swim twice a week all winter, neglect my cycling (maybe once per week at most?), and focus on running miles. After the marathon, picking up cycling miles should be fairly easy. I hope.
Hope that helps.
V