The biggest issue you are going to have is overall travel. The more spring guides/spaces you put on the shock to handle the change from one spring to the next will reduce the overall shock travel by the width of the spacer/guide. I know it doesnt sound like much but when you talk about doing three different rate springs you will most likely lose close to a 1/4 inch of travel. This will make it a lot easier for the suspension to bottom out putting undue stress on the shock tower, A-arms, and the shock body itself.

For the money you would spend on the extra spring guide/spacers and springs you could easily have purchased the progressive springs from my above post. They come in at least two different ratings, depending on your needs.

As stated before a buddy of mine uses these on his MB5 and they seem to allow for better traction, reduced chance of the chasis bottoming out off of jumps as well as soaking up bumps better dual to its dual rate abilities.

I know its fun to make things but sometimes it just not practical to follow through with them when someone else is already producing a item that is similar to what you are wanting to do and most likely will function as well or better than your design. One of the major design flaws with your idea is the use of two springs. Since you use two different springs it will be harder to keep them aligned for proper suspension travel not to mention get the return rate the same with out some special tools to measure the return rate.

Please dont take this as a knock on your idea it is simply some constructive critism.