There are multiple ways to get armor in Halo 3, some easier than others. For example, getting near 1000G in Halo 3 will earn you a Rogue helmet. Beating the campaign on Legendary is another great way but simply beating it on Normal will unlock armor as well. Getting all of the hidden skulls in the game will give you Hayabusa armor. Getting achievements will get you armor, as well. For example you can get Security armor by using the Energy Sword (I do not know the exact achievment). You currently cannot receive Recon armor unless you are a Bungie employee, but Bungie has hinted that if you unlock all of the 'Vidmaster' achievements then you are well on your way. Just remember that your password will NEVER unlock armor, even if the person claims to be a Bungie employee.
Hope that helps.