Themes

You don't mention which Drupal version you're using so I'll assume 4.5.
You can control where blocks appear in administer > blocks and setting the Region value to the desired side.

First, make sure when you post to the forums that you are more descriptive in your subject. Something like "Three Column Theme" woould be much better than "Themes".

So, to answer your question - some themes allow for three columns, some only allow for two (one for content and one for boxes). As the previous person that responded to you said, go to administer->blocks and change whether various blocks are left or right. If your choice of left or right is ignored and all blocks stay on one side _or_ the other side, then that is the way the theme you are using was designed.

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http://www.electroniclife.org/

Pushbutton (either the xtemplate version or the phptemplate version) does what you want.

I'm using it for just that sort of layout. The key is that nothing will show up on the right until you assign at least one block to the right side via your blocks admin page.

For an ad on the right, I created a custom block, made sure it was a "full HTML" block and then put in the code required by the ad-serving company I use. Assigned it to the right, gave it a low weight number and voila, a three-column site.