Its gonna have to be cut to fit your application, there are so many variables, in the end you might not end up with a true 3", could be more or less.

What you're going to need to do is make a template of a stock windshield using fiberglass mat, cloth, and resin. Using your stock windshield, wax it a few times, or you can use toilet bowl sealer as a mold release. It needs to be pretty thick so that it has some strength and so that it duplicates the window pretty good. Then, cut it to fit your new window opening (this is done after the chop is completed of course). After you've got it fitting perfectly in there (you'll want to install the fiberglass template with the rubber to make sure it fits), take it, and a new winshield, to a glass shop, they should be able to cut it for you.

Also, the way a '57 window opening is shaped, be sure that you DO NOT bend the pillars at all, because it will screw up the shape of the opening, and will be impossible to make a cut stock window fit.