Question: is odst before halo 3?
Answer: ODST. It actually takes place during Halo 2. ... While ODST did come before Halo 3, it's not really focused on Chief like 1 and 2 are (Which could throw off the feel of the game not being an invincible walking tank). But I do agree with you on that OP should probably play it chronologically, with ODST then Halo 3.