I read in the other timelines that packet 3 has to be mailed back. I was wondering if this is the same for cubans or do they just go and drop it off since the postal service in cuba is really bad.

Hey... so don't worry about that. You actually take all the required forms and documents including packet 3 with you the day of the interview. No need to mail anything. My fiance took all he needed with him that day. At some point in the day they collected that and then he waited for at least a few hours and then he was actually interviewed. Good luck with your journey!!!