db={"posts":[{id:1,title:"my title",content:"bla bla bla"}],"comments":[{post_id:1,user_id:"123",content:"great article!!!"}],"users":[{id:"123",name:"Ferb"}]}

db.posts.aggregate([{"$lookup":{"from":"comments","localField":"id","foreignField":"post_id","as":"commentsLookup"}},{"$unwind":"$commentsLookup"},{"$lookup":{"from":"users","localField":"commentsLookup.user_id","foreignField":"id","as":"commentsLookup.usersLookup"}},{"$unwind":"$commentsLookup.usersLookup"},{$group:{_id:"$_id",title:{$first:"$title"},content:{$first:"$content"},commentsLookup:{$push:"$commentsLookup"}}}])