db={"Projects":[{"_id":"A",},{"_id":"B",},{"_id":"C"}],"Episodes":[{"_id":"A/Episode01","project":"A","name":"Episode01"},{"_id":"A/Episode02","project":"A","name":"Episode02"},{"_id":"B/Episode01","project":"B","name":"Episode01"}],"Sequences":[{"_id":"A/Episode01/Sequence01","project":"A","episode":"Episode01","name":"Sequence01"},{"_id":"A/Episode02/Sequence02","project":"A","episode":"Episode02","name":"Sequence02"},{"_id":"B/Episode01/Sequence01","project":"B","episode":"Episode01","name":"Sequence01"}]}

db.Projects.aggregate([{"$match":{"_id":"A"}},{"$lookup":{"from":"Episodes","localField":"_id","foreignField":"project","as":"episodes"}},{$unwind:"$episodes"},{"$lookup":{"from":"Sequences","localField":"episodes.name","foreignField":"episode","as":"episodes.sequences"}},{$group:{_id:"$episodes._id",episodes:{$addToSet:"$episodes"}}}])