While the AMD Ryzen 9 3900X might not be a pure 1:1 competitor for the Intel i9-10900K, their similar price tier is quite enough to prompt a comparison in this case. The most important difference between the two CPUs is the core count and the boost clocks, and the latter is supposed to have a greater impact on the scores, but this is not particularly evident in the latest 3DMark scores.
https://www.notebookcheck.net/Intel-s-Core-i9-10900K-is-neck-and-neck-with-AMD-s-Ryzen-9-3900X-in-the-latest-3DMark-tests.452935.0.html