List Blog post categories and show related posts by them

I would like to know if there is a way to show the list of blog categories. when you select X number of categories, then the latest 3 (or whatever number) should show as related posts on the pages.

anyone tried this? I saw one example from a long time ago – http://wordpress.org/support/topic/plugin-posts-2-posts-filter-available-posts-by-category-actually-taxonomy?replies=6

but when i try to add that code in my functions.php file i get an error because it doesnt know what “P2P_Box_Multiple” is.

any help is really appreciated.

http://wordpress.org/extend/plugins/posts-to-posts/