Allow Comments by Default for Multiple Post Types

I have this working for one post type, but need to do it for three. I tried the “else” statements but I realized I suck at PHP.

function my_comments_open( $  open, $  post_id ) {  $  post = get_post( $  post_id );  if ( 'contacts' == $  post->post_type )   $  open = true;  return $  open;  }