input: Write a python function to combine three strings.
  assert combine_strings("Hello,", " World!", " Goodbye") == "Hello, World! Goodbye"
  assert combine_strings("abc", "def", "ghi") == "abcdefghi"
  assert combine_strings("I ", "love ", "Python") == "I love Python"
output: def combine_strings(str1, str2, str3):
      return str1 + str2 + str3