I have a three handle faucet. I replaced the washer on the end of the stem, but that wasn't the problem. What could it be?

Hi Mary,

You have a bad seat,(see image) this is the part the washer rests on to stop the flow. If it's nicked or rough the washer won't seat correctly. I put up a exploded view of a tub faucet so you can see exactly where the seat is.( the dotted lines off the washers point to them. You will need a seat wrench to remove and install the seat that's bad. If you can't tell by the temperature of the water leaking out simply remove the stem and washer assembly and run your fingernail around the seat. Replace any that are nicked or rough.
Good luck, Tom

Thank you, but I found the problem- I was missing a washer between the stem and the 'body'. Similar to what someone else reported elsewhere here about buying an assembly with a missing O-ring. (Maybe why this happened- the guy at the hardware store gave me the washer out of a packaged assembly, and then put it back on the display! I didn't say anything because I was being given the washer for free---)