While "free" external 3D plugins are rare and risky, here is the most reliable way to expand your 3D capabilities in CS6:
Adobe Photoshop CS6 actually has built-in 3D capabilities (no separate plugin needed) — but only in the Extended version . The standard CS6 does not include 3D features. Adobe discontinued CS6 years ago and no longer sells or supports it. photoshop cs6 3d plugin free download