Database
[{date:ISODate("2020-02-01T10:00:00Z")},{date:ISODate("2020-02-01T10:03:00Z")},{date:ISODate("2020-02-01T10:02:00Z")},{date:ISODate("2020-02-02T10:00:00Z")},{date:ISODate("2020-02-02T12:00:00Z")},{date:ISODate("2020-02-03T12:00:00Z")},{date:ISODate("2020-04-02T12:00:00Z")}]
Query
db.collection.aggregate([{$setWindowFields:{sortBy:{"date":1},output:{value:{$sum:1,window:{documents:["unbounded","current"]}}}}},{$set:{value:{$toString:{$add:[{$floor:{$divide:[{$subtract:["$value",1]},3]}},1]}}}}])