Stage 1 shifts a bit firmer, good for towing, RV's or mild driving etc. Stage 2 is a lot firmer, while stage 3 is either bite your tongue hard, or won't shift by itself at all.
Keep in mind, "Stage 1-2-3" is only a marketing name, like Stage 1-2-3 cams, or 1/4, 1/2 or full race cams. none of it is transferable to other kits. You're better off looking on the B&M site and seeing if you can find any relevant info there. Or Transgo is another big company. We stock the B&M kits here.