Many people find Web pages that they like that have interesting designs or structures in the HTML. It can be very tempting to save the HTML or CSS for those designs to your hard drive for use on your own site. But is this copying an "idea" (which is legal under copyright law) or a "fixed, tangible representation of an original work" (what copyright protects)?
A Good Rule of Thumb - HTML and CSS Are Protected by Copyright
If you see a design that you like, save it to your hard drive, and then replace all the content with your own, you are violating copyright. This is true even if you change the IDs and class names to make it look like your own work. If you didn't spend the time to create the HTML and CSS yourself, then you may be violating copyright.
But... Fair Use, Templates, and Coincidence
Proving coincidence could be very difficult if someone were determined to get you to change your duplicate design - but there are many 3-column websites out there that look a lot alike. If you like the design of a site, you should start by not looking at their HTML or CSS. Instead, focus on trying to re-create it yourself. If you don't copy every aspect of the design, and you write the code yourself, you could argue that you reverse engineered the design. I don't recommend this - but if you have a good lawyer, you could be safe. A better bet would be to contact the designer, and see what they think of your derivation. Most of the time, if you're willing to credit the original, they won't be upset that you imitated them.
Fair use is tricky, especially when it comes to Web pages. Most Web pages are fairly short, so any snippet of the HTML or CSS would have to be equally short. Plus, when you claim fair use, you are implicitly admitting that you infringed on the copyright. So if a judge feels it's not fair use, you're liable.