how to make 3d game

Cool! I recommend you start with a raycaster, which is an easy way of making a 3d game. It calculates the size of the object by “casting a ray” from the player until it hits the object, finding its distance.
There is a tutorial here https://en.scratch-wiki.info/wiki/Raycaster
Hope you have fun! If there's anything in specific you need help with, feel free to reach out!

There's a lot of tutorials on this topic but the most easy to understand tutorials would probably be @MathMathMath 's tutorials i would suggest this tutorial to start out https://scratch.mit.edu/projects/406896730/ it's far easier to understand than griffpatch's tutorials and there is multiple parts to the tutorial linked in the description of the project. And if you have any questions about 3D there is a variety of 3D studios where you can ask experienced 3D scratchers about whatever you might think. Hope this helped!

USE A PROJECTION MATRIX screen x = (3D x - Camera X) / (3D z - Camera Z) * FOV , screen y = (3D 7 - Camera y) / (3D z - Camera Z) *FOV

There's an entire topic for this: https://scratch.mit.edu/discuss/topic/59444/