you are using the same variable name for each session:
$userSession = $_SESSION['MM_Username'];
$userSession = $_SESSION['pass_word'];
$userSession = $_SESSION['user_name'];
at the end of the day $userSession is now equal to $_SESSION['user_name'] and there is no variable that contains the others, you need to use a unique variable name for each one:
$MM_Username = $_SESSION['MM_Username'];
$pass_word = $_SESSION['pass_word'];
$user_name = $_SESSION['user_name'];
and make sure to use the correct variable when resetting the sessions.
$_SESSION['MM_Username'] = $MM_Username;
$_SESSION['pass_word'] = $pass_word;
$_SESSION['user_name'] = $user_name;