Hello.
In cc_config.xml I have the line <use_all_gpus> 1 </use_all_gpus>.
They are visible in the device management and BM. But the tasks do not work on them.
I did a trial at Octane Benchmark and it works
This is Evga X299 Dark Motherboard (151-SX-E299-KR).
3xGPU:
- AMD FirePro W7000
- AMD Radeon R9 Fury
- NVIDIA Tesla K80
Everything is without PCIe Risers.

What to do to run tasks on them also on Nvidia Tesla K80?
In another computer I also have a Tesla K80 with Titan X Maxwell and everything works as it should.


Host:
https://sech.me/boinc/Amicable/show_host_detail.php?hostid=175977
Extract from the event log:

28.04.2022 23:05:24 | | Starting BOINC client version 7.16.20 for windows_x86_64
28.04.2022 23:05:24 | | log flags: file_xfer, sched_ops, task
28.04.2022 23:05:24 | | Libraries: libcurl/7.47.1 OpenSSL/1.0.2s zlib/1.2.8
28.04.2022 23:05:24 | | Data directory: C:\ProgramData\BOINC
28.04.2022 23:05:24 | | Running under account chr80
28.04.2022 23:05:27 | | CUDA: NVIDIA GPU 0: Tesla K80 (driver version 472.98, CUDA version 11.4, compute capability 3.7, 4096MB, 4007MB available, 4111 GFLOPS peak)
28.04.2022 23:05:27 | | CUDA: NVIDIA GPU 1: Tesla K80 (driver version 472.98, CUDA version 11.4, compute capability 3.7, 4096MB, 4007MB available, 4111 GFLOPS peak)
28.04.2022 23:05:27 | | OpenCL: AMD/ATI GPU 0: AMD Radeon (TM) R9 Fury Series (driver version 3240.6, device version OpenCL 2.0 AMD-APP (3240.6), 4096MB, 4096MB available, 7782 GFLOPS peak)
28.04.2022 23:05:27 | | OpenCL: AMD/ATI GPU 1: AMD FirePro W7000 (driver version 3240.6, device version OpenCL 1.2 AMD-APP (3240.6), 4096MB, 4096MB available, 2432 GFLOPS peak)
28.04.2022 23:05:27 | | Windows processor group 0: 36 processors
28.04.2022 23:05:27 | | Host name: BOINCPOLAND5
28.04.2022 23:05:27 | | Processor: 36 GenuineIntel Intel(R) Core(TM) i9-7980XE CPU @ 2.60GHz [Family 6 Model 85 Stepping 4]
28.04.2022 23:05:27 | | Processor features: fpu vme de pse tsc msr pae mce cx8 apic sep mtrr pge mca cmov pat pse36 clflush dts acpi mmx fxsr sse sse2 ss htt tm pni ssse3 fma cx16 sse4_1 sse4_2 movebe popcnt aes f16c rdrandsyscall nx lm avx avx2 vmx tm2 dca pbe fsgsbase bmi1 hle smep bmi2
28.04.2022 23:05:27 | | OS: Microsoft Windows 10: Professional x64 Edition, (10.00.19042.00)
28.04.2022 23:05:27 | | Memory: 63.70 GB physical, 73.20 GB virtual
28.04.2022 23:05:27 | | Disk: 930.94 GB total, 820.18 GB free
28.04.2022 23:05:27 | | Local time is UTC +2 hours
28.04.2022 23:05:27 | | No WSL found.
28.04.2022 23:05:27 | | VirtualBox version: 6.0.14

It may have to do with AMD video driver conflict.
I have to admit that you couldn't have NVIDIA and AMD video drivers run on the same computer. At the time you couldn't do that, and they fixed that some years ago.
May still have some small bug for that.