Using an expression to change focus distance three times in one composition (rack focus effect)

Hello all,

I'm running AE CC 2014. I know next to nothing about expressions, but I've been experimenting with them to try and find a way to create a rack focus effect on a photo that I cut up into several layers using Photoshop.

I'm working in 3D, and I'm changing my camera's positioning to zoom in and pan around the image. As I zoom from layer to layer, I want to create a rack focus effect by changing the value of the camera's focus distance.

I found an expression that allows me to keep the focus distance locked onto one layer, and then I found another expression that builds on the first one and allows for one rack focus between two layers after a few seconds:

target1 = thisComp.layer("people left/painting cutup.psd");

target2 = thisComp.layer("Middle man 1/painting cutup.psd");

tStart = 6;

tEnd = 7;

d1 = length(thisComp.layer("people left/painting cutup.psd").transform.position[2],transform.position[2]);

d2 = length(thisComp.layer("middle man 3/painting cutup.psd").transform.position[2],transform.position[2]);

ease(time,tStart,tEnd,d1,d2,)

My problem is that I don't know how to build on that expression and add a second rack focus to a third layer. I really don't even know where to start - does anyone have any advice?

Many thanks.