1 and 2. I couldn't answer. I've got about the same depth as you currently.
3. I've added sand - a couple times at about 10 lbs each. You can add sand from a cup, slowly submersing trying not to start a storm and pour once you reach the bottom. it'll settle out after time. As far as cycling, I don't think it would unless you're adding live sand.

But that being said, I see your sig says a 24 gallon cube. First I'm gonna assume this is the tank you're talking about and my second assumption is it's an 18" cube.

1. Do you like the way a 3" sand bed looks in an 18" tall tank, or a bigger tank?
2. Adding that much more sand is gonna reduce the water volume another 1.5 gallons/inch. Since you've already got 20 lbs of sand and another 20 lbs of rock losing an additional few gallons of water could be critical.

Just a complete side thought, if all you wanted was the depth for looks, you *could* raise everything using something like a Jaubert system, with the front edge of the plenum backed off so you could hide it with sand. I don't have any experience with the Jaubert type setup, but you could look that up. That would require tearing down the whole system. You'd get to keep some of the water volume.