I don't think you can unlock Recon armor anymore, a while ago they said they were unlocking it for players but I think that was only for the players who played around that time.

Here is what they wrote :

Part of the final transition for Halo services is unlocking Halo 3 Recon armor for all Halo 3 players. That process, which was started this morning, will take place in waves over the next few days. If Recon isnt unlocked for you today, please check back later.

So unless they are still unlocking it for player periodically, then there is no way to receive the armor (unless you know some type of modder that could unlock it for you.)

I sure wish I could get Rogue helmet :/ They did not reward me with it.