How do you play as or unlock Venom for Spider-Man 3 on the Xbox 360?

You can't play as Venom in Spider-Man 3 for the Xbox 360. The game doesn't feature Venom as a playable character. You only play as Spider-Man throughout the story.

There are no cheat codes or unlockables that allow you to switch to Venom either.

However, Venom is a significant antagonist in the game's story, and you'll face him in several boss battles.