Which CPU should I get for 3d rendering and other 3d stuff? The 2920x Threadripper or the Ryzen 3900x? Because the 2920x is much cheaper, I wonder if getting the 3900x is worth it considering the price difference (370$ for the 2920x vs 500$ for the 3900x)
I cannot speak of the 2920x but I have the 3900x. I have it OC at 4.3ghz on all cores and use it for 3d rendering using Mandelbulb3d software. It will outperform my 16 core 32 threadripper 1950x and game better with a better single-core performance.
It's not just the cost difference in the CPU you have to consider. The threadripper motherboards are much higher priced as well, and for good reason.
As far as real world performance goes, the 3900x has a leg up on the Threadripper by about 13%. It's one of the top 10 CPUs available right now as far as performance.
See this for more details: UserBenchmark
I would typically only recommend a Threadripper to someone that is doing video editing or other thread heavy tasks, because that's what it's designed for.