Database
db={"user":[{"userId":1},{"userId":2}],"doc1":[{"userId":1,"data":"1234"},{"userId":1,"data":"123461"},{"userId":2,"data":"1234135"}],"doc2":[{"userId":1,"data":"12345678"},{"userId":2,"data":"123431615678"},{"userId":2,"data":"123451351678"}],"doc3":[{"userId":1,"data":"12"}]}
Query
db.user.aggregate([{$lookup:{from:"doc1",localField:"userId",foreignField:"userId",as:"doc1arr"}},{$lookup:{from:"doc2",localField:"userId",foreignField:"userId",as:"doc2arr"}},{$lookup:{from:"doc3",localField:"userId",foreignField:"userId",as:"doc3arr"}}])