Requirements for Integrating Thunderbolt 3 Into Product
You can request info on becoming a Thunderbolt developer at https://thunderbolttechnology.net/contact/thunderbolt-developer .  It says they will get back to you within 3 weeks.  As long as you are a company and/or startup, they should at least talk to you.  It will help if you have a company email and/or website.  If you are currently buying parts from any distributor that reps Intel, you can also try going through their Intel FAE (Field Applications Engineer).