Yes, no other accessories needed. The filter can be directly screwed on your camera lens. 

Hello, please refer to the size chart or capacity display on the details page. 

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 

No. You cannot connect RF lenses anywhere but to R series Canon 

Don’t forget that it’s a manual lens adapter. Works with full manual lenses. If your lens has no physical aperture ring, you cannot control it from your camera