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 

No, this is just for the mirrorless canons 

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 

Any lens that use the M42 size screw in mount. The only other screw mount is M39 and these won’t fit, they would need a different adaptor. They will invariably be vintage lenses and you will only be able to use them in the aperture priority or manual modes, stopping town the iris when metering. Search on EBay for “M42 lens”, that will give you an idea of what you can find.