With all of the NHL teams having revealed their third jerseys in competition this year, NHL 10 can now release them as well. EA Sports has published the code unlocking them.
The code is: rwyhafwh6ekyjcmr The teams available with new third jerseys are the Chicago Blackhawks, Colorado Avalanche, Florida Panthers, Nashville Predators, and Minnesota Wild.
Third Jersey Code [NHL 10 via Operation Sports]