input: Write a python function to find the largest number among three given numbers.
  assert largest_number(3, 5, 9) == 9
  assert largest_number(10, 20, 15) == 20
  assert largest_number(7, 7, 7) == 7
  assert largest_number(-1, -5, -3) == -1
output: def largest_number(a, b, c):
      return max(a, b, c)