Works fine with 3" (actual) PVC.
I found this plug fits a PVC drain pipe with an actual (as opposed to "nominal") inside diameter of about 3".
The plug seals fine against both liquid and air. It comes from the factory with some white grease on the rubber ring, but I used plumber's grease to re-grease the rubber ring, the tapered plastic surfaces that expand the rubber ring, and the bolt and wing nut. With everything lubed this way finger tight is all that's needed to make a seal.
BUT don't lube the outside of the plastic
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ring. PVC pipe is already slippery inside, and I had to remove the lube I first put on the outside of the plastic ring.
The first reviewer is right, a stainless steel bolt and wing nut would be better, but then the plug would cost more. For temporary use the way it is works fine, for long-time use a stainless bolt, washers, and nut would of course be better.