In mild weather you can mow at 2"

If you want to keep weeds out, mow at the setting that keeps the grass the most dense. For your grasses taller is better.

An awful lot of KBG lawn problems are caused by cutting height too low. Ask me how I know. There isn't really any benefit either, this is usually done in the hope that the mowing frequency will be reduced. It doesn't work that way. 3" is fine for KBG. Scalping is often the result of low cutting height, and these damaged turf areas provide an opportunity for weed species to take hold. The best defense against weeds or undesired grass species is maintaining a dense, healthy turf. Longer blades mean the plant can store more reserves in the roots and better drouth tolerance when summer arrives. Frequent mowing comes with the territory no matter the cutting height.

Use a reasonably sharp, balanced blade (important) and vary cutting direction at 45° and 90° angles now and then . This encourages the turf to grow more upright, what happens when the same mowing pattern is used repeatedly over time the grass tends to take a set or is entrained to lay down in certain directions. Varying mowing pattern also tends to avoid underlying rutting in the soil, and presents a pleasing, finished appearance.

I have a Scotts 48" riding mower Model L2048. I always use the highest setting and lawn looks great. No weeds at all. My immediate neighbors have dandelions and because my lawn is lush and dense, they have yet to set foot in my lawn.