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

Yes, they are. I've used these in my RP and had no problems. Each battery can take over 400 photos, plus some video, before needing to be recharged 

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 

yes 

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