[Asiaplay]

Understand your comment - and no there is not a module I know of which allows for this...

Basically X-Cart does not work as easily to do a skin change live within Admin, as some other carts do (basically for X-Cart just one single style of skin template, is kept in the skin1 folder at any time - so template change = replace of many files to match the new skin style).

Assuming you have not modified pages yet (or too many pages and can easily modify them again) - then doing the skin install (install.php) and only choosing to change the skin and not a "fresh install" is safe.
It is the same as you did at the initial install, except it jumps to the last step of choosing the skin files (so there you can choose the 3-columns in the same way you did the 2-columns before). This will work without writing over any data in your store.
Note that skin reinstall, is the default option which is selected, if you run install.php the 2nd time - so not that difficult to do.
See the manual and you will understand more what I mean.

Perhaps just take a backup of any php files you have already changed before doing this - so you can more easily go back to the 2 column layout if you want to (or use those same changed files in the 3 column layout).

Cheers - Asiaplay