$user = M('user');

$b_user = M('b_user');

$c_user = M('c_user');

$list = $user->alias('user')->where('user.user_type=1')

    ->join('b_user as b on b.b_userid = user.user_id')

    ->join('c_user as c on c.c_userid = b.b_userid')

    ->order('b.user_time')

    ->select();