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

db.collection.aggregate([{"$addFields":{"Affinity":{$reduce:{input:["$Orgtype1","$Orgtype2","$Orgtype3"],initialValue:[],in:{$concatArrays:["$$value",[{$substrCP:["$$this",0,1]}]]}}}}},{$set:{Affinity:{$filter:{input:"$Affinity",cond:{$ne:["$$this",""]}}}}}])