Canon EOS 3 -- How to fire with the back open?

I'd like to visually check sync on this camera. Is there a way to get the shutter to fire with the film door open/off? Thanks!

Try setting CF-2 to 1 ("leave film leader out after rewind"). It may not let you choose a variety of shutter speeds though.

Some cameras had to be fooled into thinking the back's closed. If the back itself has a small finger on the upper, inside edge carefully* put a toothpick
into the hole in the upper light trap of the body.
Don't know if this will work with an electronic counter system that counts
sprocket holes with infrared

*the tooth pick needs to be lightly wedged to hold the sensing switch in place
but breaking it can be a PITA if the tip ends up falling into the top cover.

The CF-2 trick has been known to work with EOS cameras, though logically it doesn't make sense. I think it's a sort of "back door" Canon needed to create for repair personnel.

I thought of the back interlock switch as well, but googling the topic shows at least some people didn't have luck. Doesn't hurt to try, though.

I can't imagine why Canon thought they were doing anyone a favor by refusing to trip the shutter with the back open.

The device that plugs into the hot shoe and options the camera for a complete over ride at the system level is what is needed, rather than poking about the rear. This is a Canon proprietary device used during servicing and assessing shutter timing accuracy (among a host of system reporting lines). The EOS 1, 1N, 1NRS, 3 and 1V variants have this (the 1V has a different interface).

The annoying infrared film advance mechanism of the 3 has an interlock that prevents the shutter firing, and this is already known. If you break or damage the interlock, you will be in for a service anyway where a technician can properly and safely check this "sync" thing being mentioned.

Other than being effective for its native function (leaving the film leader out) CF-2 "trick" does not work with the EOS 1N (a sprocket-driven drive system which can be fired with no film in it, and which I have only done probably once in 23 years of ownership!), so I cannot see it working with the EOS 3 with the IR advance.

Thanks everyone. CF-2 trick does not work, so will test with film.