Jon,
The 3D waterfall plot was removed in version 3.1.8, so I suspect 3.1.7 is the last version with it. I will link it below.
The plot is gone for now, we don’t have any plans to introduce it again.
I don’t think you can install two different versions at the same time. But to get around this, I would install 3.1.7 and then simply copy the whole Spike directory at C:/Program Files/Signal Hound/Spike/ to somewhere else. That Spike directory is fully standalone and is not dependent on any other files on the system. Then simply install the latest version of Spike. You can run the latest version of Spike from the normal location and when you need the 3D plot, run it from the directory you saved.
https://relatedcontentdemo.kinsta.cloud/sigdownloads/Spike/Spike(x64)_3_1_7.zip
Let me know if you have questions.
Regards,
Andrew