Why is my Sterling three handle shower tub faucet leaking?
Remove the seats, put pipe joint compound or Teflon tape on the threads, and reinstall.  If it still leaks check threads inside the rough-in-valve (housing in wall) that the seat screws into.  If those threads are bad you have 2 options: You can use 5-minute epoxy on the threads or you can replace the faucet.