I think witcher 3 is optimized for nvidia... Can you add a cheap 650ti with an AMD card as ur main? or else you need good cpus, if you have amd gpus. But its something we knew for more than 2 years now.
Even nvidia users can gain up to 10fps extra, if they use a cheap 650ti for physix only and their gpu 770-780-970 for the rest.... U gain up to 10fps. (not in SLI mode, u cannot do that, but what you can do, is to add both cards and from the control panel, you command the 650ti to run only physx, while the other card everything else.)