yes you can
You get an achievement
You can play through each difficulty and unlock the achievement for it, playing on Legendary will unlock Hard and Normal Difficulty achievements retroactively without having to play through again
Yes, like the original achievements, you can do the same for the Mythic Achievements pack.
you have to complete halo 3 odst on legendary, have xbox live and complete halo 3 on legendary
check xbox360cheats.com ,then click h then look for halo 3 then click achievement then look through them to see if it is or not.
You have to complet the game on legendary
head-unlock overkill achievement shoulders-unloked by default with the new update body-unlock triplekill achievement
Unlock Halo 3:ODST's Campaign Complete Achievement. Once you beat the ODST campaign, visit Halo Waypoint on the dashboard to unlock the item.
You unlock the odst armor by unlocking the UNSC SPARTAN achievement
You get an achievement and if you have halo waypoint, you get the CQB helmet. Well you actually get it on heroic but you get what I'm saying. You also get an attachment if you've completed halo 3 on legendary too. Hope this helps!
You may have missed a level. Double check and if not it may have not done the levels all on legendary. Or you may have completed the game on legendary with a friend and not on your own!
beat campaign on legendary