[{difficultyLevel:"Intermediate",},{difficultyLevel:"Intermediate",},{difficultyLevel:"Intermediate",},{difficultyLevel:"Intermediate",},{difficultyLevel:"Low",},{difficultyLevel:"Low",},{difficultyLevel:"Low",},{difficultyLevel:"High",},{difficultyLevel:"High",},{difficultyLevel:"High",}]

db.collection.aggregate([{"$facet":{"Intermediate":[{"$match":{"difficultyLevel":"Intermediate"}},{$sample:{size:3}}],"Low":[{"$match":{"difficultyLevel":"Low"}},{$sample:{size:2}}],"High":[{"$match":{"difficultyLevel":"High"}},{$sample:{size:1}}]}},{"$project":{merged_results:{$concatArrays:["$Intermediate","$Low","$High"]}}}])