Hi Scott. The standard screw mount is 1/4"-20 to be exact.

The fitting you are looking for is called a T-nut, but in brass it is also simply called a "threaded insert" like here:


EDIT- Jim and I posted at the same time. He is right in that larger cameras, particularly motion picture cameras use the heavier 3/8 size. Most standard tripods for photo are 1/4"-20 though. I would like to know a source for the nut with the 3 screw holes.