[{_id:"xxx",xyz:[[1,2,3],[1,2,3],[1,2,3]]}]

db.collection.aggregate([{$set:{xyz:{"$map":{"input":"$xyz","as":"m","in":{x:{$first:"$$m"},y:{"$arrayElemAt":["$$m",1]},z:{$last:"$$m"}}}}}},{"$project":{x:"$xyz.x"}}])