This model is also called the 'Penrose Triangle'. The configuration is impossible to create in 3D, using full cubes (with all 6 sides intact for each cube). What you are looking at is an optical illusion, where only 3 adjacent-to-each-other walls from each cube are included, and the inside concave corners masquerade as external convex ones :)
Also, I used this model for the cover image of my 2004 book, 'Rendering for Beginners':
The book cover is mentioned in this article, which also has more info+links regarding the tribar model:
And, here are my alternate versions of the tribar: http://www.shapeways.com/model/672155/penrose-tribar-with-stand.html and http://www.shapeways.com/model/670170/impossible-tribar.html.