I've never been able to get these to work (and I can handle old stereoscopes without a viewer).
I can get 3 dots but I can't keep at that focal when I try to take in the full picture.
It's very important that once you have the three dots, you continue to look at the center dot until it is very sharp and clear and you can see very distinct edges. At that point, you will see that the background appears to be a few inches behind your screen or paper. When you get to that point, just follow the background up to the image.
Some people still have trouble, even after doing that. But, if you take the time to get that center dot clearly in focus, you stand a much better chance of seeing the hologram.
You have to pause before moving your eyes as your brain starts to "decode" the 3D information being fed to it. As it does the image will become clearer and you can slowly move your eyes upward.