[{"FieldA":1,"FieldB":1,"FieldC":1,},{"FieldA":1,"FieldB":null,"FieldC":null,},{"FieldA":null,"FieldB":1,"FieldC":null,},{"FieldA":null,"FieldB":null,"FieldC":1,},{"FieldA":null,"FieldB":null,"FieldC":null,},]

db.collection.aggregate([{$match:{"$expr":{"$or":[{"$ne":["$FieldA",null]},{"$ne":["$FieldB",null]},{"$ne":["$FieldC",null]}]}}}])