Use the W and S keys to adjust the focal length, which is how wide it is. Use A and D to adjust the Z-axis, which is how far from the camera it is.(The closer, the better you can see it in 3D) Use arrow keys to adjust the X and Y axis, which is how high on the screen it is and how far to the sides. Press space to have a stickman walk forward across the screen. (I like making a path for him to walk on in 3D.) N to make him go away. Press 1 to reset it.
Props to @MathMathMath for such a cool 3D tutorial. It taught me how the Z-axis works in scratch, and how to use blocks effectively. His tutorial is here: https://scratch.mit.edu/projects/406896730/ Use the sliders for precise changes, and the controls for big ones. Have fun!