3 Iron distance tris would be an overall low volume year. When you include minimal 2 week taper, and 2 week recovery for each you end up with 3 months of low volume that the big volume weeks can't catch up with. I have done a bunch of 3 IM years and they were lower volume than years with 1 or no Ironmans. This year with no racing, no tapers, no recoveries, no travel, is much higher volume. I am almost never sore, and never tired from yesterday's race or the weekend's big training or an super hard intervals on the bike and run that I feel compelled to do when racing. A lot of my intensity is in the pool with only some on the run (mainly hills) and some on the bike. When I am racing need to do way more intensity on bike and run and more single volume days on each. On a continuous base mode, you just get up and go daily to whatever you feel like doing, but you end up being able to do it all the time. I should surpass 900 hrs and I have one 21km run this year, and other than that, long runs will be 14-15km. But I'll end up with my max volume running year in the last 15 years. I only had 2-3 bike rides in 3-4 hrs range all year. Most were 30-120 minutes only. When the workouts are shorter, its easier to go out frequently and not feel fried (because you never deplete yourself) and then you can repeat it tomorrow and the next day, next week, next month.

By definition Ironman racing will also entail many low volume days, weeks and months.