[{field1:"hello",field2:"bye",field3:"good day"},{field1:"hello",field2:"bye",field3:"bad day"},{field1:"bye",field2:"bye",field3:"good day"},{field1:"hello",field2:"hello",field3:"good day"}]

db.collection.aggregate([{"$group":{"_id":{field1:"$field1",field2:"$field2"},"field3":{"$first":"$field3"}}},{"$project":{field1:"$_id.field1",field2:"$_id.field2",field3:1,_id:0}}])