Adjust the height of the blades. Models will vary, but generally there is a lever near the rear wheels to set the height of the blades. Opposite the lever is a strip of metal containing holes at various heights. Select a hole and insert the hook end of the lever into the hole. Doing this will raise or lower the wheels, which in turn adjusts the blades.
Test a patch of ground with the setting you selected. Use a ruler to measure the depth of the soil that has been loosened if you need an exact measurement.
Make the necessary adjustments to raise or lower the blades until you achieve a depth of 3 inches.
Chop into the ground with a garden hoe. Pull the hoe toward you to loosen the soil. Use a garden fork to loosen dirt when the soil is rocky.
Continue to chop or dig into the soil until you reach the desired depth.
Break apart large clumps of soil by striking the dirt clumps until the clumps split apart. Remove large rocks that have been brought to the surface.