How do I unlock Recon armor in Halo 3?
Solution 1:
It used to be that in Halo 3, the recon armor was not attainable except by working at Bungie or having a Bungie employee give it to you (source). Reasons for being gifted the armour included making an exceptionally interesting or funny level in Forge.
However, it is now possible to unlock the armour through normal means. You must have both Halo 3 and Halo:ODST, both of which much be associated with your bungie.net account. Then, you must complete all of the Vidmaster challenges, three of which are in Halo: ODST. (Source/Tutorial)