I have had a porcelain crown on tooth #3 for several years. Recently, I had to have a root canal on that tooth.
The Endodontist put a temporary filling in the tooth after completing the root canal and told me to return to my regular dentist to have the crown "repaired". What does repairing a porcelain crown entail?

I don't think a crown can be repaired, I believe you will probably have a new crown put on.