Although the final robots designs are usually edited in a 2D layout editor due to their complexity, it is often extremely useful to begin design in a 3D CAD software for mechanical engineering before exporting to 2D. These CAD programs

We usually use Autodesk Fusion 360. SolidWorks is a very similar option.

Fusion 360 is available for free for educational, hobbyist, and non-commercial use. Its website is here.

SolidWorks currently requires a paid license for research use. Licenses may be available for a fee managed by the Berkeley Mechanical Engineering department (Daniel Teal notes: contact Rene Viray in mechanical engineering; a license was $3000 per seat in 2021).