input: Write a function that takes a string as input and returns the input string repeated three times. The function should handle both lowercase and uppercase characters. If the input string is empty, the function should return an empty string.
  
  assert repeat_string("abc") == "abcabcabc"
  assert repeat_string("Hello") == "HelloHelloHello"
  assert repeat_string("123") == "123123123"
  assert repeat_string("") == ""
  
output: def repeat_string(s):
      return s * 3