I know you used to be able to get it by unlocking all the Vidmaster achievements, but I am not sure if that still works. I know Bungie gave it out to everyone but you had to get it before a certain date.Unfortunately nobody can get it now. I think it was 343's decision to give the armour to everyone, not entirely sure though.
Quote:On March 31, 2012 Bungie.net transferred all Halo 3 and Halo: Reach record-keeping to Halo Waypoint. As part of the transition Recon armor was unlocked by default for all online Halo 3 profiles.
It sounds like any profile that was online during the time should have access to Recon, it may also be applicable to anyone accessing Halo 3 online even now (I wouldn't be able to check as I already have it unlocked). It is very possible that completing the Vidmaster achievement will still unlock it.
I should put the game in so I can run around for a bit and have a little nostalgia moment with my Recon pretending while my head's alight with blue flame.