[{"_id":1,"Region":"Northern","Orgtype1":"A1","Orgtype2":null,"Orgtype3":null},{"_id":2,"Region":"Northern","Orgtype1":"A4","Orgtype2":"B2","Orgtype3":null},{"_id":3,"Region":"Northern","Orgtype1":"B5","Orgtype2":"C1","Orgtype3":"D0"}]

db.collection.aggregate([{"$addFields":{"Affinity":{"$switch":{"branches":[{case:{$regexMatch:{"input":"$Orgtype1","regex":"^A"}},then:["A"]},{case:{$regexMatch:{"input":"$Orgtype2","regex":"^A"}},then:["A"]},{case:{$regexMatch:{"input":"$Orgtype3","regex":"^A"}},then:["A"]}],"default":"none"}}}}])