You just need to have your faucet repaired. The middle stem is the diverter stem. To repair it you don't need to shut off the water to the valve although it would be better to repair all three stems. Take the handles off, the escutcheons off and remove the stem. Usually you need a set of deep set sockets called tub wrenches to reach back into the wall. When reassembling be mindful that there are two sets of threads on each stem. You need to keep the stem free moving while tightening the stem body.