spring relo=shorter shocks?

If it is half way compressed with it on all 4's, it will be in the middle of its travel. In other words it won't bottom out too early or top out too early. If you stick a shorter shock in there without taking any measurements, you could end up with too short of a shock and it will max out when you launch the car and won't allow full planting of the rearend which could cause wheel hop at launch. Also, keep in mind that you only relocated the springs in, not up; the shock is probably not compressed a lot more than it was prior to you moving leaf springs. 3" moved in is probably only approximately 2" in compression of the shocks.