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EOS-FX,You can use Canon EOS EF / EFS Lens on a Fujifilm FX Mount Camera body,, Your canon ef 75-300 1:4-5.6 is canon EF mount lens and fuji x-a3 is X mount camera body. so the adapter ring fit for your camera and lens. but pls notice, your lens is telephoto lens,we do suggest you could use one telephoto lens bracket that could be attached to the tripod to increase its stability. because the canon lens could not adjust aperture,so suing our adapter ring could not adjust aperture 

This is a Canon to micro 4/3 systems like the current Panasonic and Olympus models. Unless Nikon has the same mount, it will not work. 

I dont believe so since i bought it for a Canon 80d. I didnt buy it for a Nikon camera. You will likely need a different adapter 

Yes, it is indeed Nikon f mount.