3" class 5, 3" sand, power tamper and lay away. Fill in between the stone and tamp again to work in the fill sand and it should be good to go. Some old timers put chicken wire under the stone, if it's all the same thickness, to prevent uneven sinking. I am redoing a brick patio at my Mom's this summer that has been pretty stable for 42 years and it has chicken wire under the bricks. It has only a couple of small sunken spots.
Most of the flagstone work out here is layed on a bed of morter and grouted in with morter. Also, the flagstone is random, I.E. not cut in squares. We score them with a 4-1/2 grinder with a diamond blade and shape to fit.
I'll post a pic of my shower, same principal. (This is slate).
I had a friend place flagstone on top of my regular concrete porch. We wanted a more rustic look so we went for rougher pieces of flagstone and thus have more variation in elevation between individual pieces due to the fact that you can't sink or fill under the stone to make it perfectly level. Not really a problem for us, but there won't be any roller skating on the thing
Basically they placed the stone in a bed of mortar and then "grouted" the spaces between the stones with mortar.