There’s a thing on instructables.com about how to make a 3D camera.
It’s a cheap solution, but I see one problem.
Note here, because the red eye has the extra reflections, it will be as though that eye is a few inches further back than the blue eye. (Imagine taking the path the light follows, and straightening it out.)
This will be close enough for taking pictures of things far away, but if you want a close up, that difference will be a problem.
So what would be better? One simple version would just mean taking two identical cameras and connecting them side-by-side:
What are the draw-backs? You need two identical cameras. If the cameras are too wide, then they will be further apart than your eyes would naturally be. The settings on the two cameras need to match one another. (Aperture, exposure time, zoom) If you are shooting stills of moving things, you’ll need to be able to fire both cameras simultaneously. If you’re shooting video, you’ll need to sync up the footage.
(You can angle the cameras together, as I did in the picture. If you do so, angle them so they converge at a distance close to your subject of interest. Anything further the point of convergence will recede into the image, and anything closer to the camera with appear to reach out of the image.)
If you need your cameras closer together than their width permits, aim them towards one another and put two mirrors between them at 45 degree angles.
Since you have a single reflection for each camera, you’ll need to mirror them in post.