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!

I think you were on the right track when you tried to replace the washer and the seat. That is generally the total fix so maybe it's time to have another person have a try at it.