run an ultra in early spring, then a 70.3 in summer, then PR a marathon in fall?

It's doable, and I would start w/ a run focus for sure. In 2013, I trained for a 50 miler that I ran in May (Collegiate Peaks Half). Vast majority of my training was running during the winter (although I relied on some swimming while dealing with a strained calf muscle). After the race I started to focus more on cycling but still managed to PR a half-marathon in early August. I then did a 70.3 in October (Austin), and I was on track for a 70.3 PR as well. Unfortunately, I crashed the bike at about 50 miles so it didn't happen.

The ultra will give you plenty of base which means you won't need to run as much leading up to the 70.3 and you can integrate some swimming as a "recovery" workout during your ultra running as well. After the 50 miler, I'd switch to a bike emphasis. I think the bike will present the biggest challenge for you. After the 70.3, you'll be able to switch back to a bit more running for the Marathon. Personally, I think a lot of the challenge in Marathons is "mental." Best of luck!

I think a lot of it depends on how well/fast you recover from the 50 miler. You really want to be careful ramping up training again after events marathon length and longer. Some people recover quickly, others it takes awhile. You should certainly have a fantastic base after the 50 miler to start training for the 70.3. How strong is your swim and bike currently?

Are you trying to be competitive in these three events or just for personal satisfaction? If its the latter, I wouldn't sweat it too much and just do what makes you happy.

Certainly possible to complete the three events, but "optimal" training for each of these is pretty different and you won't have much time between recovery from one event and the build-up to the next.

One suggestion........have you thought about finding a fall ultra instead of a spring one? Most people can do fairly well at the 50-mile distance if they have a strong marathon base, so it may make better sense to push the 50-miler out a month or two after the marathon.

From my experience, putting in the time for a 50-miler and the typical long runs required tend to make meaningful bike training for HIM or longer distances pretty difficult. It can be done, but it won't be easy.