OK, I did it, surprised myself with that…
3 gang strip plug (3way plug)
Accessed my dev account: https://account.smartthings.com
My Device Handler > Create New > From Code
Pasted in the code from here: https://github.com/LAMS144ai/smartthings/blob/main/zigbee-multi-switch.groovy
Pressed create and published for me
Plugged in the device, turned it on. On the phone app went to > add new device > smartthings > outlet > Outlet (STS-OUT & Others)
Found the device and called it ‘Outlet’.
Then back in the developer page accounts https://account.smartthings.com went to My Devices, scrolled down to ‘Outlet’, clicked on it, down to the bottom to edit it > ‘Type’ > Scrolled all the way down to the bottom of the list and found LAMS ZigBee Multi Switch.
It wasn’t obvious it had worked at first. I think I may have then deleted the device off my phone app because I couldn’t figure out where to control the other 2 plugs on the strip as Outlet only changed one. But I found the other two hiding at the bottom of my app view on the phone as Oute2 Oute3.
Seems to all work. Thanks for the code and the instructions.