add content to a specific category post

I want to add a read more link automatically in a certain category. I added the following to function.php, but no go

add_filter('the_content', 'custom_category_text');

function custom_category_text($content){
    global $post;
    $custom_category_text = '<!--more-->';
    if(in_category('3')){
        $content = substr($content, 0, 300) . $custom_category_text . substr($content, 300);
    }
    return $content;
}