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

Yes. Compatible with Canon 600D and works well 

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 

K&F have a list of which adapter will work for each lens to camera combination they support 

H, We checked your lens is 67mm, you can confirm your lens size mark"ø", if the size is right , you need to order 67mm filter.