It scales really well in terms of raw multi-threaded performance. In that metric, 3990X is total beast.
But I still have some qualms about my 3990X. I don't know how to measure it well without having all the Threadrippers of same generation, but I suspect the 3960&3970X might be better in what I would call "mixed-threaded workflow", something like Interactive Rendering, which is multi-threaded (rendering part) and single-threaded (UI re-draw and select&manipulation) at same time.
All 3rd-gen Threadrippers have the same IO chip (connecting the physical dies, 3, 4 and 8 respectively, to memory channels), same amount of (4) memory channels and same amount of power limit (280W TDP).
I feel that 3990X already stretches thin within these limitations.
In theory, 3990X would ideally be 8-channel chip (which exists.. the 3995WX Threadripper Pro), with much more robust IO (Looks like Zen4 Genoa will bring that) and 500W power limit (you can unlock such with powerful cooling).
But maybe it's bullshit, and nothing really helps in these cases. And maybe 3dsMax (And Windows) is just unsalvageable in their core and no amount of super-powered hardware will make the experience fluid under all circumstances.
Of course, if you don't have your own sizeable render-farm, or aren't using cloud services, 3990X is the best deal. But otherwise I am almost partial to consider 3970X to be the best deal.