1. It had to be Legendary
2. It had to either be weak to dragon or die instantly to a move as strong as DM
3. It does not resist electricity
Therefore after looking over the list of legendaries I have come up with 3 possible options for his 3rd pokemon
1. Palkia- Water/Dragon means no electricity resist and is weak to dragon
2. Rayquaza Dragon/Flying means "
3. Deoxys-A Absolutely no defense whatsoever