Can you get god mode in halo 3 if yes then how?

If you meant to become invincible, there is a piece of equipment you can pick up in Campaign called Invincibility. It's dropped by Brute Chieftains and renders the user invincible for 12 seconds.

The other way to become invincible is edit the settings in Forge or Custom Games. You can set your level of health, your recharge rate, your level of shielding, your ammo, your movement, and your damage. For health, damage, and ammo, you can set all of those to infinite if you so please.