input: Write a function to calculate the average of three numbers.
  assert avg_three_nums(1, 2, 3) == 2.0
  assert avg_three_nums(10, 20, 30) == 20.0
  assert avg_three_nums(5, 15, 25) == 15.0
output: def avg_three_nums(a, b, c):
      return (a + b + c) / 3