It's not just the 3000 series. I have an RTX 2080 Super and I have exactly the same symptom. Doesn't matter whether I pick the GPU or the Optix:GPU (not sure which I should be using?). It also doesn't matter whether I render in Poser or use the Queue manager.

In Poser 11, I had this problem if I rendered a multiframe animation directly in Poser, it always used the CPU. But in Poser 11, single frame renders worked fine, and animations were also fine if I sent them to the Queue manager. Now in 12, it's all CPU.