If you just want to subtract 1 from the layer’s z value you could do it like this:
thisComp.layer(“Layer 1”).position – [0,0,1]
The easiest way to express a value as the three individual coordinates would be to do something like this:
p = thisComp.layer(“Layer 1”).position;
[p[0],p[1],p[2]]
It’s all just JavaScript array notation.
Dan