В твоем коде исправил ошибку.
{exp:query sql="SELECT screen_name, member_id, (total_forum_topics + total_forum_posts) AS total_post FROM exp_members ORDER BY total_post DESC LIMIT 3"} <a href="{path="member/{member_id}"}">
{screen_name} всего - {total_post} </a>
{/exp:query}
Мой рабочий вариант. Просто лидер по ответам.
{exp:query sql="SELECT screen_name, member_id, total_forum_posts AS total_post FROM exp_members ORDER BY total_post DESC LIMIT 3"} <a href="{path="member/{member_id}"}">{screen_name} всего - {total_post}</a>
{/exp:query}