How do I set the conditions to do x action -once-?

Try using the condition: Trigger Once while true

So when a second one is built and the conditions are met, will that run again or how does that work? I saw the trigger once while true but it didn't seem to be the solution for future conditions that would need to be met. Again, only asking as you may see something I don't.

You could also try giving each powrplant a boolean called IsActive. It starts false, but when the plant gets activated by three workers, you set it to true.

Then, whenever you run a check to see if a powerplant should get activated, also check if IsActive is false. So you'll only ever activate plants that have false booleans.