Slider Face Morph

I am trying to design a UI element, a slider, that will allow the user to control a gradual morph of their face. 

What I'd like is for the one side of the slider to morph to face mesh 1,  the middle to look like the user, and the other side to morph to face mesh 2.  The finished product should be a smooth dissolve between all three cameras controlled by one slider.  

So far, I have had some success.

I've gone through the visual scripting basics, and also I've looked at:

https://arbootcamp.com/snapchat-intermediate/opacity-slider

and

https://support.lensstudio.snapchat.com/hc/en-us/community/posts/4407641428244-Drag-to-switch-between-two-face-masks

I still can't find the right script to trigger the post effect I want, which is to use a slider to dissolve between three render targets.  

Thank you.