def append_x(x, a=None, b=None, c=[]):
    a = a or []
    b = b if b is not None else []
    a.append(x)
    b.append(x)
    c.append(x)
    print(a, b, c)

s, t, u = [], [], []
append_x(1)
append_x(2, s, t)
append_x(3, s, t, u)
print(s, t, u)