low, medium or high z3?

Great question.

As you are suggesting, you can let the duration dictate the intensity.  You should also consider the event you are training for and the stage or period you are at in your training cycle.

If you are in the mid to late part of your build-up training for an event lasting more than a couple of hours, and especially if it lasts more than 5 hours or if you are early in the base training cycle for shorter events, even down to 3-4 minutes like a skimo or XC ski sprint have found the most benefit from doing a higher volume of low Z3.  Staying at this lower intensity allows for more volume and quicker recovery. Evoke coaches have had good results using these principles.  This Z3 training supports for higher intensity work in short events and is the even specific work for longer events.

If you are in the later stages training for a shorter event like a VK or 2-hour skimo race, then training in high Z3 will be useful as it supports the Z4 work needed for these sub 2 hor events that are either steady state for less than an hour like a VK or for 2-hour races with undulating terrain like an XC ski or Skimo race.

We like to build a pyramid of support. where each layer of intensity supports the one above and is supported by the one below.

This is where coaching becomes much more art than science as each athlete is different, and you MUST pay close attention to recovery.  You don’t just do intervals on Tuesday because that is what the plan says.  When it comes to high-intensity training, there is a lot of nuance and individualization.  That is why there are so many different approaches.  I am merely telling you what I have used for decades with good results and by extension Evoke coaches typically use.

I hope this helps.

Scott