As another test, if you use your lens at the widest aperture (f/2.8) on all three cameras, does the problem still exist? If not, it's then highly likely it's indeed an aperture blade in the lens that is failing.
Also agree that the likelihood of three cameras all having failing shutters is extremely low.
While it is unlikely that three cameras have failed shutters, the additional info seems important.
Now it seems you are saying that one camera has a problem and one doesn't. And it happens with two different lenses.
That would seem to be a camera problem.