When I add a triplet in simple (by typing a note and pressing 9 to activate the triplet) the triplet function stops once I have filled in the first triplet. I don't have to press anything special to stop the triplet.

If you are encountering this problem, you must be activating your triplets differently.

I agree with J Adrian ^ - it sounds like the actual tool in the Simple Entry palette is being activated (as depicted below).

If this is the case, the shortcut you can use to deactivate it is CTRL+9 [on the number row] (Windows) and CMD+9 [number row] (Mac). One little nuance is that if you use the shortcut once and then don't release the CTRL or CMD key and just press 9 again (or use the shortcut in succession too fast), this could deselect the note duration and the keyboard shortcut won't work until you select the note duration again. So, just make sure that you have a note duration selected if this keyboard shortcut is not working!

Thank you Lawson and J A V. I didn't understand that unlike all the other durations, pressing 9 should be done after the first note of the triplet.

The CMD 9 works for me on OSX. 

If I'm going back and adding a note as a chord to a note that is within a triplet, I must not use the triplet palette item, or the 9, correct ?

If you are just adding a not to an existing note in the triplet and are using Simple Entry (set to the same duration as the note), just add the note; the triplet makes no difference.