Millerlite said:3 inches is not deep enough at all, substrate should be deep like 8-10 inches more of you can. Not only will it e moist underneath if prepped right it will help hold humidity and a next for a box turtle is roughly 6inches deep sometimes my female will dig deeper or less deep, all depends but giving them the option is the key. People think turtles are like hey I have eggs lay. When of fertile a female will take her time and are very picky with nesting. Twigs, not moist enough, too moist, not deep enough, rocks , all can lead to a abandon laying spot and on to another spot. Deeper substrate is the way to go.
I absolutely agree.
Be very careful with the underground mesh. Don't use the one with the large holes...chicken wire? Make sure it's tiny holes, like a screen. My friend used chicken wire, and her enclosure wasn't deep enough and her small box turtle got caught in it. They thought she escaped, and never saw her again, until years later when they renovated her yard, and found the shell, and decomposed turtle caught in the chicken wire.