So you want to have for example only one line of text in the title in the Recent Ads widget?
If so then the CSS code above i pasted will do that, the CSS (as far as i know) cannot count the number of words, but it can hide the text which does not fit inside a container.
If you want trim the title with PHP then you can create your own versiof of the Recent Ads widget, to do that:
1. copy the wpadverts/includes/class-widget-ads.php to your theme or child-theme folder.
2. in the new file change the class Adverts_Widget_Ads
to something else for example class My_Adverts_Widget_Ads
, change the values in __construct() function and modify the title display in widget() function.
3. finally in your theme functions.php file add the below code
add_action( "init", function() {
include_once dirname( __FILE__ ) . '/class-widget-ads.php';
register_widget( "My_Adverts_Widget_Ads" );
} );
This will register your own custom version of the Recent Ads widget.