Halo 3 is a first-person shooter video game. It tells the fictional events and settings in the Halo universe at the end of the Halo trilogy. It was developed by Bungie Studios and published by Microsoft. Halo 3 was released on September 25, 2007 for the Xbox 360. - Summarized from Halo Nation.
At the top of the page of the armour variant of your choice, just hit the orange download button for a zip file of the Pepakura, Foam and 3D models that exist for it. You'll want the Mark VI as the base armour and then the different helmet, chest and shoulder attachments.