The body/mount/mirror situation on the EOS 3 is exactly like the 5Dii, so you can find a lot of info on the net about which lenses work. I picked up a $12 adapter from B&H. Works fine, but no auto-focus confirm. It is worth buying it to see if you really want to mess with this proposition and buy an expensive adapter. The screen is not designed for manual focus, so it can be a little difficult to focus depending on your eyes. The Takumar 50mm 1.4 will not work. I suggest the 55mm 1.8, which is an awesome lens. Given it costs like $25 it is a no brainer. I have both and think the 55mm is better. 28mm f3.5 is another great cheap lens.