I have an old house with an existing sub panel on the second floor that has only a three wire with ground feeding it. I just had a home inspection that said the panel was not properly grounded/bonded. The ground wire is attached to the neutral bar with the common and other ground wires. I'm assuming this is wrong. I don't have the ability to run any new wires to this panel. There is not a separate ground attached to a grounding bar in this panel. I can only work with the ground from the main panel that is part of the 3 wire. Is there a configuration that I can use that will pass code?

What you have is no longer allowed.

You will need to check with the local inspector to see if what you have can remain, which should be allowed.

He can tell you how he would want any additional grounding, if any, done.

Sounds like you have 3 wires plus a ground already in place. (can't believe a home inspector actually opened a panel) Anyway, if that is your wiring configuration it can be rectified by adding a grounding bus and separating the grounds and neutrals. BUT... if this is an existing installation in an old house, you shouldn't have to change it to meet currents codes, since it met the code at the time of the installation. Just my opinion.