[{postId:"232323",post:"This is my first post",commentsOnPost:[{commentId:"232323_8888",comment:"Congrats",repliesOnPost:[{replyId:"232323_8888_66666",reply:"Thanks",likesOnReply:["user1","user5","user3"]}]}]}]

db.collection.update({postId:"232323"},{$push:{"commentsOnPost.$[c].repliesOnPost.$[r].likesOnReply":"user2"}},{arrayFilters:[{"c.commentId":"232323_8888"},{"r.replyId":"232323_8888_66666"}]})