If you have FTP hosting, you definitely can. With FTP hosting, you are also free to switch to whatever editor you like. If I was to try to change your page as it is to three columns, I think I would find it much easier to just create a new page (and then copy content there) than to try and change the existing page. I suspect that so will you.
If you are not sure about how to do that, look around for a nice three column template.