Claire – nothing like this should happen. Looks like you just tried this through the visual editor and selection for replace did not happen right. Can you just switch to HTML view and try it- should go through fine without issues and you will need to change the threecol class to fourcol to help span the extra space created when one of the pricing tables is deleted.
If you still have trouble, pls reply as a private message here with the site URL and a temporary login to the WordPress dashboard and I can take a look. You may also email me the same (see bottom right box in my profile page).