diverter valve leak - 3 valve diverter valve?

My shower has three knobs and the one in the middle diverts the water flow from the tub spout to the shower. I think that means I have a 3 valve diverter valve, if not please correct me.

When I turn the diverter knob so that the water comes out of the shower, there is still significant water flowing from the tub spout. There is also a leak coming from the diverter knob itself.

I have tried having someone replace the washer and the seat, but this hasn't helped. Please tell me what the problem could be and how to fix it. Thanks!

diverter

The diverter is double seated, meaning that it shuts off one section when it is fully "open" and the other when it is "closed". Replacing the washer only fixes one way, and probably not the way that was leaking. The best way is replace the diverter stem, because few places will have the parts to repair the second seat's seal.