There are damn few types and brands of firearms that I haven't fired, and I have over 70 years of experience with almost any you could name. I've written a book and more articles than I can count.
I can tell you unequivocally that .38 super and .38 ACP cartridges WILL chamber and fire in .357 magnum revolvers. The cases for the .38 ACP and .38 super are identical - the only difference is the loading. The case rims are not as thick as those of the .38 special/.357 magnums, and they are also smaller in diameter.
I don't own a .38 super gun, but I do have an antique .38 ACP Colt, and have some .38 ACP rounds for it. As an experiment, I loaded some of these in my Model 28 - no problem, although there was a bit of slop fore and aft due the thinner semi-rim on this round. I fired a cylinder full of these, and although the empties ejected somewhat irregularly, all was well. In a pinch, lacking proper .38 special or .357 ammo, it's good to know that there are a couple of other rounds out there that will work.
No conjecture here - I tried it. It's definitely possible.
By the same token, the rule is to always use proper ammo for your gun, and this is not to be taken as a recommendation.
Here is a picture of a .38 ACP and a .357 magnum chambered side by side in a Model 66 to compare.
John