Success, I finally managed it - by editing my keycommands_de.json file.
a) As I am using a German keyboard layout I assigned [Shift-Ö] for activating basic triplets.
b) To end Tuplet input I use just the [Ö] key command.
c) If I want to get to the Tuplet popover I use the key command [Shift-,]
b) and c) can be configured in Doricos preferences.
a) can be configured only by editing the keycommands_de.json file.
I added these lines:
{
"NoteInput.StartTupletRun?Definition=3:2" : [ "Shift+Ö" ]
},