I suggest using the Advanced Custom Fields plugin for this. You can use it to add other entry fields to a post type (in this case, you’d probably want to add it to the Page type). In your case, you would probably want two other Wysiwyg Editor fields so you can write with formatting.
The trick will be that, if you assign the new fields to the Page post type, all pages will have the two extra fields. You may want to create a new post type and assign the fields to that.
Another way to go about this would be using something like the Chunks plugin. This allows you to have small snippets of content that you can call into a theme whenever you’d like. Using this would allow you to have saved pieces of text you could use in one of the columns, and also include that text elsewhere on the site, if you’d like.
Also, one further idea: if you just want the content to flow into three columns and it’s mostly going to be text, you could use CSS3’s column-count. This doesn’t work in all browsers by default, but there are cross-browser shims like css3-multi-column.js which can fix that for you.