input: Write a function to concatenate three strings.
  assert concat_strings("Hello, ", "World", "!") == "Hello, World!"
  assert concat_strings("Python", " is", " fun") == "Python is fun"
  assert concat_strings("I", " love", " coding") == "I love coding"
output: def concat_strings(str1, str2, str3):
      return str1 + str2 + str3