A Grass/Fairy Gaia Legendary, A Flying Apollo Legendary, and Dark/Fire Hades Legendary make up the trio with a Electric/Fighting Zeus Legendary as the leader of the trio.
Why would you have Gaia, Apollo, Hades, and Zeus? Gaia is a Titan, grandmother to Zeus, Hades, Poseidon, Hera, etc... I like the idea of a Greek-inspired game, but let's make a logical trio, here. Zeus, Poseidon, and Hades should be the main legendary trio, with either Kronos, Gaia, or Oranos as the trio master. I'd probably go with Zeus as an Electric/Flying (or just straight Electric) ram or eagle, Poseidon as a Water/Ground (because earthquakes and the ocean) horse (because Poseidon created horses, yo!), and Hades as probably.... Ghost/Fighting 3-headed-dog? --- When life's really getting you down, take solace in the fact that you're going to die.