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

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