The demo actually just shows 1 Featured Posts widget that is set to show 3 posts horizontally. The posts show horizontally because the css is written to have the posts/div’s float next to each other. So the reason your posts are stacking is because you are using 3 multiple (separate) widgets rather than 3 posts from 1 widget (does that make sense?).
We can still make this happen though. You can still have separate widgets float next to each other to appear in the same way as the demo, but they will have to be adjusted in your css to do so. If you can give me a link, I can help get this going once I take a look at your site and the code.