Can you play Halo 3 online for free

No, you can't play Halo 3 only for free. Having Xbox Live Gold lets you play online but its $8 per month. They give you an option though if you don't want to pay and they will offer you a FREE 1 month trial. This is allowed 1 time up to 3 accounts as long as you use different emails.

I Hope This Helps - Henry