He's holding all 3 keys. How would he have gotten the one the Autobots had unless he got into their base. This would be a brilliant way for Starscream to return to the cons. Just gives Megatron the keys and remarks on how Dreadwing has failed him at every turn whereas not only Starscream has retrieved all the keys for a reborn Cybertron but the location of the autobot base. Megatron would be foolish not to acknowledge Starscream's worth and accept him back.