Hello,
I had the same problem with my WR3M rework station on channel #1.
After searching, I found the flaw.
The V8 varistor that is connected between the pin that takes the signal from the HPA200 iron button and the ground, as shown in the attached image. The varistor has low insulation. It didn't close short, but its resistance was around 7K ohms. While the others have 220k ohms.
As I don't know the correct voltage for it, I removed it from channel 3 and soldered on channel 1, because channel 3 I leave the soldering iron straight and not this line is not used.
After that the wr3m is operational!
Cheers,
BP