Convert the following classes into dataclasses such that the initializers that the dataclass generates have the same behavior as the regular class:

class A:
def __init__(self) -> None:
    self._length = 0

class B:
def __init__(self, x: int, y: str = "hello", values: list[int] | None = None) -> None:
    self.x = x
   self.y = y
   self.values = [] if not values else values

class C:
def __init__(self, a: int = 3) -> None:
   self.a = a
   self.b = a + 3