input: Write a function to merge three arrays into one.
  assert merge_arrays([1, 2, 3], [4, 5, 6], [7, 8, 9])==[1, 2, 3, 4, 5, 6, 7, 8, 9]
  assert merge_arrays([10, 20], [30, 40], [50, 60])==[10, 20, 30, 40, 50, 60]
  assert merge_arrays([1], [2], [3])==[1, 2, 3]
output: def merge_arrays(arr1, arr2, arr3):
      return arr1 + arr2 + arr3