The query on the mashup is showing category 8, not category 3. I assume that’s OK but I mention it because the shortcode you listed above shows category 3.
You can test the query itself (without MapPress) by putting this in a template file like sidebar.php or single.php:
If the results look incorrect try deactivating *all* other plugins and switching to a standard theme for a moment – they often interfere with queries.
If that doesn’t help please send a login and I’ll take a look at the site. It’s best if you can replicate the problem on a test site or a copy of the site, rather than using the ‘live’ site.