[{"id":"1","vec":[1,2,0]},{"id":"2","vec":[6,4,1]},{"id":"3","vec":[3,2,2]},]

db.collection.aggregate([{$unwind:{path:"$vec",includeArrayIndex:"i"}},{$group:{_id:"$i",avg:{$avg:"$vec"}}},{$sort:{"_id":1}},{$group:{_id:null,avg:{$push:"$avg"}}}])