[{"courses":[{"course":"Btech","semesters":[{"sem":1,"subjects":[]},{"sem":2,"subjects":[]}]},{"course":"MBA","semesters":[{"sem":1,"subjects":[]}]}]}]

db.collection.update({},{$addToSet:{"courses.$[c].semesters.$[s].subjects":{"subject":"C++"}}},{arrayFilters:[{"c.course":"Btech"},{"s.sem":1}]})