Apparenty, this is a thing. I've have a bunch of 3rd Gens but never encountered it.

GaryS, here's the similar problem you may have been thinking of...

Difficulty removing slide on 3rd Gen? (w/ photos)

I finally got the slide off. I should have whacked it with a rubber mallet from the start like a few of you suggested. But instead removed the safety (while the slide was still on the gun.) I have a feeling that was quite unnecessary.

But eventually the slide loosened up and came off. There was a little residue of some sort on the inside of the frame, but the issue was probably the bottom of the safety/slide hanging up on one or more of the three levers. Not sure which one.

The slide hung up going back on the gun as well. Again, friction on the levers. The two levers on the right don't seem to depress down into the frame as easily as my other 3rd Gens. Stiffness possibly explained by it being a new gun. Running it wet with a good break in might resolve it.

Now I can enjoy my previously unfired 3913... that is, until it's time to strip it for cleaning.