The first cut in the Spring should be done at 2.5 inches. This will remove a lot of grass and open the root crown to the air and allow the soil to warm up faster. This is done around mid-April.
Your second mowing should be done at a height of 3 inches. By your third mowing you should be cutting your grass to a height of 3.5 inches. This allows the turf grass to fill in nice and thick helping prevent weed growth and promoting deep root growth.