Combining more than two displacement maps (Wrinkle maps)

[darrenwallace3d]

Hello,

I have a character that will need multiple displacement maps for wrinkles  driven by blendshape values.

There is a base(neutral) displacement map and then multiple wrinkle maps for different parts of the face and at any one time many need to be triggered without affecting the each other. The maps are from Zbrush so they all have the base displacement within them.

I found a solution online which works perfectly for two maps. But when I try to daisy chain this method the wrinkle maps start to override each other. 

In this example image the desired result is having 3 maps combine to make 1,2,3 appear on the sphere. 1 is the base map, 2 and 3 are the additional maps. Both 2 and 3 images have 1 (neutral) map within them. 
The top part of the image shows 2 maps being combined with the desired result. 1 + 2 but without 1 being applied twice. The lower part is my attempt at daisy chaining on the 3 map. I assumed the result of how 1 & 2 are combined would now be treated as the new base map and combining 3 onto that would be the same. But it appears to override it. I'm not sure if I'm doing something wrong or if Maya is being silly. I tried this with both Vray and Arnold incase it was a renderer issue but both give the same result. Any advice would be greatly appreciated, thanks.