With the THC button on the screen?! But we have always used the UCCNC and it has an M code to turn the THC on/off. You will have to ask machsupport about how to do that in mach3.
I hoped it was clear I know how to turn THC on in Mach3. I also hoped it was clear that none of the methods I stated work when using the plugin for the UC300ETH.
I got the plugin from your site so I was hoping you may have some insight into why none of the available methods in Mach3 appear to work.