Read More link under Excerpts does not appear at all

gene, I've had the same issue with Event Calendar 3...

The problem is that Event Calendar 3 actually removes the wp_trim_excerpt filter and replaces it with its own filter called ec3_get_the_excerpt (check out line 595 in eventcalendar3.php).

Unfortunately, this is just a bad design decision on the part of the plugin's author in which a core Wordpress filter gets removed.