input: Write a function to determine which number is the greatest among three given numbers.
  assert three_way_comparison(1, 2, 3) == 3
  assert three_way_comparison(10, 5, 7) == 1
  assert three_way_comparison(4, 9, 6) == 2
output: def three_way_comparison(a, b, c):
      if a > b and a > c:
          return 1
      elif b > a and b > c:
          return 2
      else:
          return 3