Shut off water behind tub at valves. If no valves, shut water off at meter, and drain down. Open a laundry tub faucet or anything lower. Remove the handles and eschuchteons. Remove the stems with an adjustable wrench. If the stems are recessed in tiles, you need to buy plumber's sockets. Remove the brass screw that holds the washer on. Remove and replace washer. Lube stem with plumber's grease. The middle stem (diverter) will be a different washer. Inspect this washer for wear. Inspect the seats inside the body of the faucet. Remove and replace if you see wear. When you re-install the stems, make sure the cap thread gaskets are in place. Install plumber's putty behind eschutcheons. Turn on valves and check for leaks.