Who do I talk to in order to film How High 3?

The Execs/Producers are the ones who pick the people on a project.

Ideally the film studio/production company/companies. Whoever owns the rights. But unless you're a well-known producer with a great track record, or have an A-list attachment, or unless you can pay hundreds of thousands to millions of dollars, no company is going to let you use their franchise or sell you the rights to it. It's not worth it for them. If you film without owning the rights, you will get sued.

Since I assume you don't have any of the above, I highly recommend that you forget about the idea. Also because it's much more complicated and complex than I roughly described. If you want to film something get your own script, with an original franchise.