Yeah, there are Platinum Medals. Out of the three games, only two of the games have them (Battle for Naboo and Rogue Leader).
In "Battle for Naboo" you have do better than you normally would for gold medals. There is one big catch behind these medals, they don't tell you exactly how much better you have to be in order to get them. I have the Platinum requirements written down and can post them if you like. Your reward for all plats is the AAT and Swamp Speeder.
In "Rogue Leader" the Plats are also referred to sometimes as Ace Medals. They aren't as hard to get. First, you have unlock ace mode by getting all gold medals. Then, simply complete a level with ace mode turned on to recieve them. Your reward for all plats is your final rank, "Gallactic Allied Commander".