Pluto's three existing moons are Charon, Nix, and Hydra. Charon was the ferryman of Hades. Nix was the goddess of the night, and mother of Charon. Hydra is named after the nine-headed serpent that Hercules battled. Pluto itself is another name for Hades, god of the underworld.
I voted for Cerberus, the three headed dog that guarded the gates of the Underworld, and his two headed 'brother' Orthrus. I'd love to hear what names you guys have voted for!
The official ballot can be found here: http://www.plutorocks.com/