[{"objectId":"aaaaa","userId":"userId1","processedAt":"2021-06-01T12:16:49.349Z"},{"objectId":"bbbbb","userId":"userId1","processedAt":"2021-10-11T12:16:49.349Z"},{"objectId":"ccccc","userId":"userId1","processedAt":"2021-10-25T12:16:49.349Z"},{"objectId":"eeeee","userId":"userId1","processedAt":"2021-09-12T12:16:49.349Z"},{"objectId":"fffff","userId":"userId1","processedAt":"2021-09-28T12:16:49.349Z"},{"objectId":"ggggg","userId":"userId1","processedAt":"2021-09-23T12:16:49.349Z"},{"objectId":"hhhhh","userId":"userId1","processedAt":"2021-07-23T12:16:49.349Z"},{"objectId":"iiiii","userId":"userId2","processedAt":"2021-09-29T12:16:49.349Z"},{"objectId":"iiiii","userId":"userId1","processedAt":"2022-01-29T12:16:49.349Z"},{"objectId":"iiiii","userId":"userId1","processedAt":"2022-02-29T12:16:49.349Z"},]

db.collection.aggregate([{"$match":{userId:"userId1"}},{$sort:{processedAt:-1}},{"$addFields":{"month":{"$substrCP":["$processedAt",0,7]}}},{$group:{_id:"$month",last:{$first:"$$ROOT"}}},{$sort:{_id:-1}},{"$replaceRoot":{"newRoot":"$last"}},{"$project":{month:false}},{$limit:3},{$sort:{processedAt:-1}}])