[{"_id":"1","array_field":[{"data":[{"regions":[{"result":{"item":["4","5","3"]}},{"result":{"item":["5"]}},{"result":{"item":["1"]}}]}]}]}]

db.collection.aggregate([{"$project":{newField:{"$map":{"input":"$array_field","as":"m","in":"$$m.data.regions.result.item"}}},},{"$unwind":"$newField"},{"$unwind":"$newField"},{"$unwind":"$newField"},{"$unwind":"$newField"},{"$group":{"_id":"$_id","newField":{"$push":"$newField"}}}])