Some consider a Series 3 marginal for a 400/2.8. That historically lead you to a Series 5, although now you can split the difference with a Series 4.
Due to the added weight and bulk of a Series 5, that would lead one to a dual tripod setup, one primarily for the 400/2.8 and one for general use with lighter lenses.
Most people shooting a 400/2.8 on a tripod probably sooner than later end up using a gimbal like a full Wimberly. And that tends to lead to a dual tripod setup simply to avoid constant head swapping as you change shooting modes (400/2.8 vs more general shooting).
But it can and is done with a single Series 3, so as I said, it complicates the decision. We would have to know much more about your shooting subjects, both with a long lens like a 400 and shorter lenses, relative frequency, and etc. I know that is tough since you do not even own the lens nor imminently plan on a purchase?
You may also consider that a 400/2.8 is a very special lens, pushing the 10K. It may be better leave it out of the decision and then deal with it if or when it comes about. You might not, for example, want to end up hauling a Series 5 around for use with a 70-200 because some day in the distant future you may get that lens