Figured it out !!
Using the triple ID (MyTriple) returned by getFirstTriple or getNextTriple use
_plex3.EnumTriplesBySource(MyTriple, 0, ref MyList);
getFirstTriple(MyList, ref MyTriple);
getTripleTarget(MyTriple, ref tgtTRPId, ref isTrp);
getObjName(tgtTRPId, ref tmpStr1);
tmpStr1 then will contain the qualifier