I was working on this, and I thought it was interesting enough to share. Use W, A, S, D to move horizontally, Z and C to move vertically. Press Q to look at sliders to mess around with. X 1 and X 2 change the position of the left and right sides of the cube, allowing you to change the position and size of it. Y 1 and Y 2 are basically the same as the X 1 and 2 variables, except they're on the Y axis instead of the X axis. Z 1 and Z 2 are wierd. Z 1 changes the position of the cube, but not just one of the sides. Z 2 changes the depth of the cube.
It's not actually 3d as you can't look in different directions, only in one. It also isn't a cube even though that's what I'm calling it, it's a rectangular prism. Still, it might be useful for something like an isometric game or something. Also, you may note that the vertical controls are awful. I wanted to use Space to go up and Shift to go down but Scratch does not allow you to read the shift key. So I had to choose different keys instead Also the cube does not like the sides of the screen