Hi,
HikaShop 3 is a major version of HikaShop. It's the biggest update version since the first version of HikaShop.
We gave 1 month of beta test time so that anyone could install it on a copy of their website and report any issue so that we could fix it before the final release.
No one reported such 1 column issue in that time frame...
Also, for such important update, you're supposed to test it on a copy of your website, not directly install it on your live website.
You can't expect that to work. It's like if you update Joomla 2.5 to Joomla 3 and you expect it to work without changing anything. It's impossible.
We're doing our best but we can't fix what we don't know about.
And yes, for that 1 column issue, you should be able to fix it by editing the CSS files in the configuration page of HikaShop, looking for the CSS code Jerome gave in his message and removing it. It should be easy to do even without knowing how CSS works.