@Pattaya_Web: that's how the default WordPress gallery template looks too, the code in the default template was more or less copy/pasted from WP :)
The easiest way would be to add something along these lines to your theme's CSS, or create a custom File Gallery template with these same lines in 'gallery.css' file:
#content .gallery.columns_3 .gallery-item
width: 30% !important;
margin-right: 3% !important;
how you want to style the columns is, of course, all up to you. this code will force a 30% column width, with a 3% right margin.
here's the help file section about custom File Gallery templates: http://skyphe.org/wp-content/plugins/file-gallery/help/index.html#your_own_templates
let me know if you get stuck :)