Hi Brian
Try this:
layerIndex = (index-1);
xPos = thisComp.layer("expression").effect("location")("Point")[0];
yPos = thisComp.layer("expression").effect("location")("Point")[1];
spacing = thisComp.layer("expression").effect("spacing")("Slider");
groupSpacing = thisComp.layer("expression").effect("group spacing")("Slider");
groupSize = thisComp.layer("expression").effect("group size")("Slider");
layerSpacing = Math.floor(layerIndex/groupSize)*groupSpacing + layerIndex*spacing;
[xPos+layerSpacing, yPos];
I have added a couple more sliders. If you want you can simply set groupSize to 3, but I thought it was fun to have it as a slider.
All the best
Jakob Wagner