This is a free 3D transition for FCP which Thomas Graziani has created and entitled 3D Flip. Links after the break.

Here's the link to the 3D Flip.  You have to register to download the template and you may need to switch the site out of Italian.

This HD template is Motion-made: complete with drop zones that enable you to add your content in a very simple way. Once saved as a master template, the effect will appear in Final Cut Pro.

 Thomas is an Italian filmmaker, video editor and DOP.