How to put ”a3”:”b3” inside the {} after a?

Question

Dic = {”a”:{”a1”:”b1”,”a2”:”b2”}
Dic[”a3”]=”b3”
print(dic)
Gives

{”a”:{”a1”:”b1”,”a2”:”b2”},”a3”:”b3”}
But i want

{”a”:{”a1”:”b1”,”a2”:”b2”,”a3”:”b3”}}