by downloading a skin that already has 2 sidebars.

http://skins.b2evolution.net/

I am very satisfied with my actual skin, ( www.faraway.au.tt ), despite that it have only one sidebar.
It should be only some change in any of the css files and the main.php, shouldn´t it?

Still if you download a 3-column skin you'll see how it gets done. You are correct in that you will change your _main.php and your style sheet to accomplish this, but the specific changes required will depend on exactly how the nifty corners skin made itself be a 2 column skin. Thus if you download one or two 3-column skins you will see the techniques and have your 'aha' moment about how to accomplish this for yourself.

Given that nifty corners is a fixed width skin you might want to look at ality because it is a fixed width 3-column skin, but I can't say for sure that the method used in ality will easily port to nifty corners.

thx a lot, i will give it a try.