It depends. Certain suits or parts of armor are unlocked for getting achievements in campaign, finding skulls, and whatnot. Some are obtained through the multiplayer, either from ranking up or getting an online achievement.
you need to acheive different tasks in order to unlock diffferent armor. You also need to get achievements
Unlock what head?
get all the skulls easily u can you tube it also u can play multiplayer online to unlock items getting halo ODST u can unlock recon by doing the vidmaster challenges in halo odst and halo 3
Unlock it where? It's already in it by default.
You unlock the armor.
you have to complete halo 3 odst on legendary, have xbox live and complete halo 3 on legendary
you find them,look for videos on Youtube I believe that you have to find them in different levels and speaking of halo 3 do you know if you can make scarabs in forge in any places?
There isn't a known cheat code for this on Halo 3.
Your work for them.
No
Unlock Halo 3:ODST's Campaign Complete Achievement. Once you beat the ODST campaign, visit Halo Waypoint on the dashboard to unlock the item.
for finishing the halo 3 campaign on heroic you unlock the E.V.A helmet, plus the shoulders and body plate for certain missions.