Hi. I have a problem with hot and cold water mixing or so I've been told by a few plumbers. One said to replace all the shower and sink faucets as they were old and that would be the easiest place to start. We've replaced all but one shower faucet. The other two that we replaced now don't have any hot water. I don't understand why the showers wouldn't have hot water but the sinks will have hot water. Any ideas on why this is happening?

It sounds like you disturbed some crud built up on the inside pipe walls and the showers sucked it up into the valve and now the hot water inlet ports of the cartridge are clogged. Shut the water off to the valve, open it up and clean and clear the cartridge hot water inlet ports. You should still have the installation instructions with a exploded view of the valve to help you. Before you reassemble the valves turn on the hot water for a moment to flush out the supplies.
Good luck, Tom

I will try this. Thank you!