I don't have a 3-toed, but I beleive they eat more veggies than my ornate would. I think I have read 60% veggies and 40% insects is appropriate for them. I feed my ornate pretty much a 50% veggies and 50% insect diet. Like I said though, I don't have a 3-toed so maybe someone else can answer this better.
As for humidity, you can just create a humid hide and they can choose what they need. Humid hides are simple to make. Just get a big enough container they can fit in. Like a ceramic pot on its side. Then you can either put moss or sterilized dirt or something like that in it and mist inside of it daily. The other thing you can do is to put a cup with marbles or gravel or small stones in it and fill it up with water and put that in the hide towards the back. I think I also read somewhere of someone glueing a sponge to the top and just keeping the sponge wet. This will create an area with 100% humidity and they can go in and out of it as they need. Also making sure they have access to a clean water dish where they soak themselves as they want is important. I also soak my box turtle regularly.
Here is a good box turtle site: http://www.boxturtlesite.org/bxbook.html