[{"_id":ObjectId("5ce10ffb0a9531d98ad11819"),"_course":ObjectId("593107b273790c30c4e08b02"),"users":[{"level":17,"_user":ObjectId("5c08be10d1a0c91e4c739629")},{"level":16,"_user":ObjectId("593107b273790c30c4e08b07")},{"level":5,"_user":ObjectId("593107b273790c30c4e08b08")},{"level":17,"_user":ObjectId("593107b273790c30c4e08b05")}],"acr":"MH"},{"_id":ObjectId("5ce10ffb0a9531d98ad1181a"),"_course":ObjectId("593107b273790c30c4e08b02"),"users":[{"level":9,"_user":ObjectId("5c08be10d1a0c91e4c739629")},{"level":6,"_user":ObjectId("593107b273790c30c4e08b02")},{"level":5,"_user":ObjectId("593107b273790c30c4e08b07")},{"level":4,"_user":ObjectId("593107b273790c30c4e08b07")}],"acr":"MA"}]

db.collection.aggregate([{$unwind:"$users"},{$sort:{"users.level":-1}},{$group:{_id:"$_id",users:{$push:"$users"},_course:{$first:"$_course"}}},{$addFields:{users:{$slice:["$users",3]}}}])