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

db.collection.aggregate([{$group:{_id:null,a:{$push:{$arrayElemAt:["$vec",0]}},b:{$push:{$arrayElemAt:["$vec",1]}},c:{$push:{$arrayElemAt:["$vec",2]}}}},{$project:{a:{$avg:"$a"},b:{$avg:"$b"},c:{$avg:"$c"}}}])