I use 2 inches of spagnhum peat moss topped with an inch layer of cypress mulch. But several others members have their own way that works very well also
I've used coco coir as shallow as 4" and as deep as 7-8". Then I top it with a pile of sphagnum moss in various sections, around plants or in the corners. Some of mine will burrow and cover completely. Most of them sleep on top.
4" is what I tend to stick with for depth.
In a closed chamber I'd go with 2-3 inches. In an open topped enclosure I'd go with at least 4". There is no reason you can't use 4" in a closed chamber too.
It also depends on the substrate. Which one have you chosen?
I notice lately that a lot of people are choosing to use several different substrates at the same time. Just for my own curiosity, can you tell me what you are using three different substrates? I'm not saying its bad or anything. I'm just wondering about it.
Cypress mulch for its moister hold. Dirt for easier digging and a gap filler. Moss for more moister holding in the hide box.
I would just use cypress but I feel it's not steady for baby feet and the dirt to just firm it up a little.