I associate overtraining with an inability to attain a high HR. Been there, did that.
IME terrymorse is correct. Lots of moderate hours will increase your aerobic ability. Don't worry about the high end. Those 3' intervals are too short to overstress your anaerobic ability. You don't say how many weeks you have to train, which might make a difference. To do a "moderate" training session, keep your breathing rate below the level where you start breathing more rapidly. I suggest trying 130w for at least an hour per session, preferably working up to 2 hours steady. If that's too much or too little power, figure out what's correct for you. When you start dong this, you'll probably see your HR rise by the end of the hour, even though power has been steady. Gradually this HR rise will become smaller or disappear entirely. It's called "heart rate drift." And your HR at that power will gradually decrease or you'll find you can use more power at that same breathing rate, recommended. That HR drop or power increase is what you're looking for.
Probably TMI, but this study is a good look at male vs. female hearts:
https://www.frontiersin.org/articles...22.831179/full