What is the best way to build a slab foundation on a slope?

There are many ways to build a slab on grade foundation on sloped building lots, but doing three separate slabs isn’t necessarily the cheapest option. Did you specifically want a 3 level house or is that something you have chosen simply to adapt to the terrain? I ask this because if its the design you want then go for it, but it may be more cost effective to adjust the lot and built a single slab. Let’s look at both options – 

To build on 3 levels you will most likely need to pour the bottom slab, build a retaining wall as you suggest, or a short ICF knee wall. Then pour the next one, the next retaining wall /knee wall, and again the next slab, so it could be a long and labour intensive procedure. Each step would likely need to be poured individually, so that means 3 trips with concrete trucks and pump trucks, and that will add significantly to the cost. 

8 feet over 72 feet is not too extreme, so if you weren’t married to the 3 story design, it would likely be a more simple solution to just level the lot first. You can do that by removing some of the back soil and pulling it forward and build a retaining wall at the lower section. Is that feasible? That way you do one slab with 3 side ICF wall (or partial wall). Is that making sense?