I compiled Ethminer https://github.com/ethereum-mining/ethminer/blob/master/docs/BUILD.md and it works. But I did have to change ethminer/libethash-cuda/CMakeLists.txt to ignore old (sm_30-35 that don't work with CUDA 11) and add new 86 architecture (list(APPEND CUDA_NVCC_FLAGS "-gencode arch=compute_86,code=sm_86")).
What hashrate does Ethminer give you for the 3080?
86.5. For 2080 Ti with +300 mem OC it's 52.7.
Was 86.5 mh the fastest you could get with Ethminer?