The edge of "round": Three half-megameter moons

Part of the definition of a planet is a solar system body's roundness. Big things in the solar system are round, because their self-gravity is strong enough to overcome the inherent strength of the materials that they're made of. It helps that big things also tend to be relatively warm, with heat left over from their formation and from the decay of radioactive elements; warm substances are not as strong as cold substances, especially if ....