I did a one inch shackle lift and have saggy stock springs and I am able to fit 31s no problem. Remember a shackle that is two inches longer then stock is a one inch lift. So you would need shackles that are six inches above stock for a three inch lift. If you want more then an inch for cheap do one inch lift shackles and one inch body lift.
I would also suggest to get boomerang shackles. You will pay a little more, but it helps with the approach angle and helps keep the axle in the right location. Tons of places make them, just make sure they are good quality. You don't want to skimp on something that holds your axles.
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