To make a clan in Halo3, I would suggest that you rather not.
Why? Let me list it out:
1. People will laugh at your clan name if it is silly
2. People will laugh at you if you don't have the skills
3. Many clans have been developed in Halo universe since 2003
4. Keeping the clan active is a tough job, if you have less than 10 members.
5. People will spread the word about the clan you made and make false remarks about it.
6. The possibility of people being active to your clan is very limited.
7. To be honest, why would people join clans anyway....