When you collect all of the skulls in Halo 3 you get the awesome Hayabusa Armor that's the only Armor you unlock in Halo 3 by collecting skulls.
Note that you can't collect the skulls on the Easy difficulty it has to be on Normal difficulty or higher and also note that I have found all 13 skulls and Bungie put them in Bizarre places I'd recommend going to the related link to find them especially the I Would've Been Your Daddy(IWBYD) skull it's the hardest to get.
By getting skulls.
get all skulls
multiplayer achievements and skulls
Yes, you can actually. There are 13 skulls in all. If you collect all of the skulls you will get hyabussa armor.
archon armor
Get all Gold & Silver Skulls.
Get some skulls dummy
You have to collect all of them in order to get the armor for your soldier
No, in Halo 2 you can not unlock armor.
You have to find all the skulls in campaign mode.
The best places to get armor is to beat the campaign, collect skulls, or through online game play.
when get achievements (like beating the game on Legendary or finding all the skulls) you can unlock some of the armor.