Database

[{"year":2020,"grade":1,"seccion":"A","id":100,"name":"pedro","notes":[{"curse":5,"block":1,"score":{"a1":5,"a2":10,"a3":15}},{"curse":5,"block":4,"score":{"b1":10,"b2":20,"b3":30}}]}]

Query

db.collection.aggregate([{$match:{"grade":1,"seccion":"A","year":2020}},{$set:{"score":{"$filter":{"input":"$notes","as":"note","cond":{$and:[{$eq:["$$note.block",3]},{$eq:["$$note.curse",5]}]}}}}},{$set:{score:{"$cond":[{"$size":"$score"},{"$first":"$score"},{score:{}}]}}},{$project:{/** projection*/"grade":1,"seccion":1,"name":1,"id":1,"score":"$score.score"}}])

Result