With lens off camera look through rear element. Do you see anything other than a clear view through?
Set the Av to f/8.
Look through the lens and cycle the DoF button. See anything amiss?
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.
I no longer have the two earlier 70D cameras that had problems. The 90D started having problems last night. I have a used 70D backup which is working fine right now. I just want to figure out if I'm doing something to cause this.
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.
09/26/2023: New firmware updates are available.
08/18/2023: Canon EOS R5 C training series is released.
07/31/2023: New firmware updates are available.
05/31/2023: New firmware updates are available.
05/18/2023: New firmware updates are available.
03/30/2023: New firmware updates are available.