hi sejtraav,
adding a third column to serenity would require doing quite a lot of structural changes to the template. essentially what is involved is adding another div to the index.php (after the content div).
Then adjusting the content div's width, floating the #right div and replacing the faux background image on the #main div.
Once you have done that you would need to add the necessary css to float the right div.
Alternatively, if you are not that familiar with css, then rather take a look at our commercial templates that already have 3 columns.
Hope this helps
Cheers,
Chris