I find it odd that 3 bodies would do the same thing. That makes me suspet the lens since the bodies fire without a it.

Fire the lens off body? Is it giving resistance when rotating the pins to fire it? Is the shutter closing properly when fired off camera? Do you have access to other lenses to try?

Take a look at the leatherete on the bodies? Does it look like someone opened the bodies, DIY repairs? You can peel a corner back n look at the screws to see if they look tool damaged at all.

One more thing, and I know this has nothing to do with 3 bodies having the same problem... are the mirror bumpers gummy n rotted?