When the jaws of a non adjustable 3-jaw chuck become worn, they can be at least partially restored by recutting the clamping surfaces.
There are commercially available fixtures for this, but these are probably beyond the price range of most recreational machinists.
The above diagram illustrates one way that a simple fixture can be made, from common flat plate, to allow the jaws to be loaded.
Use steel plate of at least 1/4" thickness, and rough cut the diameter to the same size as the chuck. A hole should be put in the center, approximately the same size as the through hole in the chuck. The three holes for the jaws to protrude through would have to be placed next, probably by making a paper or cardboard template.
This can then be slipped over the jaws as shown, and the jaws rebored. Most chuck jaws are hard, but not so hard that they can not be bored with a carbide tipped boring bar. If a grinder is more convenient, use that.
Remember that this will make the jaws "perfectly" true on that lathe and at that diameter.