3 spacers?
I think you mean washers.
If so, only one goes on the Diff Nut. The other 2 go on the other side to hold the thrust balls with your Black grease.
As for how tight a diff needs to be, it all depends on track surface and conditions, but you want to make sure it doesn't bark! If it barks, it's too loose. You want to tighten it to just where it doesn't bark, and then give another 1/8 to 1/4 turn and it should be set pretty well. From there you want to adjust your slipper properly, which again depends on track type and conditions. But general rule of thumb is to tighten it enough so it's difficult to spin the spur while holding the rear wheels and then set the car on a table while holding the rear wheels and squeeze the throttle. You want the front of the car to lift about 4-5" before you place on the track to see how it reacts. If it's too tight loosen the slipper a touch, do the opposite if too loose.
If the slipper feels right but you're having trouble turning with the car pushing through corners, even coasting off-throttle you'll want to tighten your diff just a hair at a time. This is the same for every car running a Ball Diff and slipper.
Be sure to get all your HobbyPro/PR Racing parts from CRPplus.com.
You wont find better customer service, support or knowledgeable help anywhere else.
Rick is the best… Period!