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

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 

Nope, it’s to adapt to the smaller Nikon 1 (CX) mount of the 1 series camera, not the F mount (your D3100) on crop and full frame Nikon’s. 

Hi, it will work on a7iii, tested. 

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