Hi Torpex77,
I created a new post on the topic and a few members chipped in and we solved it 
https://community.openhab.org/t/pushover-service-in-oh3-broken/113937/13
The clunky syntax that Pushover used for sending in OH2 seems to have changed and appears more simplified. Check out the new Binding guide here.
[Pushover - Bindings | openHAB]
Most significantly, I had been typing the syntax code into the wrong script code window.
The UI rules and Scripts in OH3 offer TWO different script language systems to put your code into. I had been blithly choosing the ECMAScript javascript code window (cos I had been doing everything in java rule script with VScode in OH2) when I should have used the Rule/DSL script code window. This is because I was creating my rule in the UI this time and I guess the new engine is DSL based? Anyway, coding & scripting is painful and not my thing really.
Hope this might help you out too.
Denis