Same as in autostereogram, the trick is to look to the distance.
Close your eyes and imagine a mountain far away or some distant object, notice how your eyes adjust to see it. Open your eyes and try to look at this imaginary mountain while the image is in front of you.
When you see the third Image, treat it as if its a distant 3d object somewhere on the horizon.
When I brought an early autostereogram in to school in the early '90s my high school Physics teacher refused to try it as he thought it sounded impossible. He thought we were all in on it as we 'got it' one after another.
