Hi. Although this will not solve your issue, you can always alter the output of K2 by creating a K2 template. You can read more on this at getk2.org/documentation/tutorials/174-templating-with-k2-and-the-concepts-of-sub-templates .

i know i can costume the template and put my new version on template-name/HTML

but i don't know how to tell the template to make the first 3 leading content to behave the same for all published and not just the first 3...

Your template override produces incorrect HTML markup. That's why the CSS rule is not applied to the last item. Check your template's override code.