I thought QSK might have been talking about an IATA or ICAO code...
Thanks for the clarification Fris B.
Yes I was referring to the official ICAO designator. I seem to recall that for flight planning and ATC flight strip purposes the official ICAO designator for a Trislander in the '70s and 80s was "BN3" to distinguish it from the twin-engined BN2.
However, as Fris B has pointed out, those original designators have all changed now.