[{_id:21353456,product:"xy",text:"asdf",reviews:[{username:"User1",userID:12,text:"hi"},{username:"User1",userID:12,text:"hi"},{username:"User2",userID:123,text:"hi1"},{username:"User2",userID:123,text:"hi1"},{username:"User2",userID:123,text:"hi1"},{username:"AUser3",userID:124,text:"hi1"},{username:"AUser3",userID:124,text:"hi1"},{username:"AUser3",userID:124,text:"hi1"}]}]

db.collection.aggregate([{$unwind:"$reviews"},{$group:{_id:"$reviews.username",userID:{$last:"$reviews.userID"},reviewsNum:{$sum:1}}},{$match:{reviewsNum:{$gte:3}}},{$sort:{_id:1}}])