If you are part of an accredited higher education institution, you can get a free 12-month license for non-commercial use. This is the closest thing to a "free" full version and can be renewed annually as long as you remain eligible.
You can go back to any step in your creation process and change it without losing later work.
If you are looking for an "Adobe Substance 3D Designer Free Download," the situation is a bit nuanced. While there isn't a completely free, permanent version for everyone, there are legitimate ways to use it at no cost depending on your status.