I've trained for races before, and even between different disciplines there is a lot of common ground. The principles of athletic training for all endurance sports are pretty universal, so a lot will carry over from running to swimming for example. But this is my first time training for multisport, and there have been some phenomena that are entirely new.
The first is how tiring training for a triathlon can be. Even when I was preparing for a marathon and doing 20 mile runs on Saturdays I was never this drained. I think it's because between swimming, biking, and running, you are using every major muscle group, so more of you gets worn out than would for the same amount of training in just one sport.
The second is that training for multisport is like juggling- you can't ever let a ball drop. As I've been doing more running and biking, I've inadvertently been doing shorter and shorter swims. The result is that my swimming fitness has really dropped, and I need to spend more time in the pool to get it back. This balancing act is so difficult because as I mentioned in the last paragraph, it is so easy to over-train. You have to practice a sport at least every second or third day to just maintain fitness in that thing. And if you want to improve you have to do hard, focused workouts. But doing that for three sports at a time means that you are doing hard workouts every day, sometimes twice a day! Your calf muscles may be getting a break when you swim, but your heart and lungs are going just as hard so they never get a chance to recover.
The best solution I've found is to make sure that most workouts are at a low intensity, and only go really hard every other day. The other key is getting plenty of rest, so take Saturday AND Sunday off after a heavy week of training and make sure to have a rest week every three or four weeks.