Personally, I would go for the DC one, if only because of better IV and a different moveset (makes it more interesting to me).
Honestly, if you use these as attackers, they are all pretty usable at current levels. Also, one option is to keep the 2nd one at current level and use it to prestige.
Personally, I love Dragonites but find myself not using them that much as attackers (there always seems to be a better matchup option available). I tend to stick them in the back of gyms.
All in all, based on how you play, I would say that any Dragonite is worth maxing out because of the high CP. At worst, it will sit in the back of the gym. Start with the 3rd one if you plan to use it as a defender or keep going with the 2nd one if you want to use it as an attacker.
Right now, based on where you live, 3000 CP will likely get you to the back of the gym. So Level 30 may be the ticket.