So you have no female Platies?
That's why then. Males don't usually get along that well, especially in that small of a tank, with no females. They will get aggressive towards each other, because they have no females to mate with, and see each other as threats. Naturally, the bigger faster male is going to be dominant, so that's why your poor Guppy is being terrorized. Can you possibly upgrade the tank? Or move the Platy to another one?