First, keep your posts in a single thread, instead of starting a new one for every post. (just type into the 'Quick Reply' box at the bottom of the page) Otherwise, no one can follow it. You stated that you've swapped solenoid, diaphragm, and valve cover. The only thing left to swap is the valve body itself. That is where the problem is, and if you aren't up to fixing it, you buy another valve.
When you swapped solenoids, that should mean you took the solenoid from a nearby valve that you know is working, and put it on the faulty valve, without disconnecting any wires. So, if the number 3 solenoid that works on valve number 3 can't make valve number four work (while activating zone 3 on your controller) that will eliminate the solenoid as a problem. If you're changing the wire connections when you swap solenoids, then you haven't eliminated a wiring fault as the culprit.