Trying To Solve This Riddle Cannon T3 EF To Fd Lenses

There are EOS camera to FD lens adapters that work for macro only (due to the lens register difference you mentioned). If you already have a fantastic Canon FD (or FL? or R?) macro lens, it might be worth a try.

I have one of these adapters (used to attach a Canon FD/FL bellows), but almost never use it. I use my Canon T6s mostly with a Pentax M-42 bellows, which is quite nice and allows for easy attachment of miscellaneous lenses to the front standard.