CSSing tanzaku into 3 columns

Hey!

I have rather simple question which is a shame to post here, but i can’t solve it by myself cause i am not expert in divs, css etc.

Tanzaku theme, by default, have left and right margins. I want to fill them:
left – by menu <?php wp_list_pages(‘sort_column=menu_order&title_li=’); ?>
right – with banners (new zone) <div class=”r_b”><?php get_sidebar(‘3’); ?></div>

At 1st look all seems to be ok. But when i start to resize browser and my window become smaller, grid covers up my left and right columns.

I really dunno how to avoid it. M.b. anyone, who already made the same, can link site where i can view source?

Thanks in advance for any answer.

https://www.ads-software.com/extend/themes/tanzaku/