?
3 nodes are only required if the load is such that 2 nodes are needed to handle expected load
That's pretty much it!
Keep in mind, it's tough for us to generalize things because there are so many different usecases and usages levels. But generally speaking, LB of ProGet is about managing peak usage -- like, when a few developers run their npm builds (requesting thousands of packages) using their 32GB/8-core desktops, that can quickly overpower a server.
One server is often not enough for situations like that. Two is probably good enough. Three is pretty good. This is why we say dropping from "probably good enough" to "not enough" is not HA.
Also, it's almost always easier and more cost effective for organizations to add another server in the mix than it is to spend the time determining how much they might actually need and risks that not being enough.