Wait, why would win 10 reserve 20% gpu resources on all 3gb cards in a rig? shouldn't windows do that only to the primary gpu?
Microsoft don't care, memory is reserved on all cards. More memory - more is reserved. This memory is not used by W10, but it is not available for any programs. Another beautiful gift from Microsoft.
I still can mine ETH in W10 on 3GB nvidia cards with latest Claymore miner with 0% dev fee, but I think it will stop work after epoch #185, you can check it with "-benchmark" option.