Generate a new Python class which extends some existing class, but with a modified constructor.
class A:
    def __init__(self, param1, param2):
        self.param1 = param1
        self.param2 = param2
class B(A):
    def __init__(self, param1, param2, param3):
        super().__init__(param1, param2)
        self.param3 = param3