I am dealing with the same thing. My diverter wasn't diverting. I tried to pull the diverter stem out and it was not coming out. I damaged it trying to gorilla it out. Next project is to put a hole in the wall and cut the whole mess out and put a modern single handle valve in there. 3 handle plumbing is no longer code in California (but grandfathered in)

Thank you bro

Why do all the vids skip over swapping out the diverter valve ? They merely show a new knob.

Do you need to shut the water off first?