Fixed Shader with multiple AlphaBlended Layers ?

I'm trying to make a shader that takes 3 textures with alphas, and 3 numbers(Range 0 to 1) and draws a material made up of the 3 layers stacked on top of each other with the 3 numbers used to control the global alpha of that particular texture. This images is a visual example of what I'm hoping for:

In code, this is the closest I've got so far with a fixed shader:

 Shader "Custom/Unlit - 3 Layer - AlphaBlended" {
 Properties {
     _Tex1 ("Layer 1 Texture", 2D) = "white" {}
     _Tex2 ("Layer 2 Texture", 2D) = "white" {}
     _Tex3 ("Layer 3 Texture", 2D) = "white" {}
     _Fade1 ("Layer 1 Alpha", Range(0,1)) = 1
     _Fade2 ("Layer 2 Alpha", Range(0,1)) = 1
     _Fade3 ("Layer 3 Alpha", Range(0,1)) = 1
 }
 
 SubShader {
     Tags {"Queue"="Transparent" "IgnoreProjector"="True" "RenderType"="Transparent"}
     LOD 100
     
     Lighting Off
     Cull Back
     ZWrite Off
     Blend SrcAlpha OneMinusSrcAlpha 
     
     Pass {
         SetTexture [_Tex1] {constantColor(1,1,1,[_Fade1]) combine texture * constant}
         SetTexture [_Tex2] {constantColor(1,1,1,[_Fade2]) combine texture lerp (texture) previous}
         SetTexture [_Tex3] {constantColor(1,1,1,[_Fade3]) combine texture lerp (texture) previous}
     }
 }
 }

This is close, however, you'll notice that I'm not using the "constant" on the second and third layer to fade-out the texture. If I could write it the way I think it should be the second and third SetTexture would be:

 SetTexture [_Tex2] {constantColor(1,1,1,[_Fade2]) combine texture lerp (texture * constant) previous}

Unfortunately, multiplying 2 sources in the argument of a lerp is not allowed as far as I can tell. Anyone got an idea of how to do achieve this? fyi, this is intended for iOS devices.

Thanks for any help, Mike