The right to modify a work can be enforced through copyleft. But the copyleft of Creative Commons’s Attribution Share-Alike (BY-SA) licence, which is intended for artistic and design works, applies only to the finished object, not to any preparatory work. This would mean that it would not be an effective copyleft for projects where you need to ensure access to the designs for the finished object.
To create an effective copyleft for a 3D object (where that object is copyrightable), apply the GNU General Public License to the designs for it. Declare the object to be the “binary” and the design to be the “source”. Then anyone who receives a copy of the object can request the designs under the terms of the GPL.
Using the GPL rather than BY-SA ensures that anyone who receives a copy of the object can receive a copy of the designs for it (where the object is covered by copyright) without which the GPL is only as strong as BY-SA. The GPL does mention associated rights, so design rights might also be covered, but these do not apply as globally as copyright. But where a Creative Commons licence might otherwise apply and where access to designs is important (which is probably everywhere), the GPL is a better alternative.
(I am not a lawyer, this is not legal advice.)