The Wizard Of Karres by Mercedes Lackey

A down-on-his-luck trader suddenly finds himself responsible for three young girls from a mysterious world whose uncanny psychic abilities mark them as witches; as they flee across a crowded, politically tangled interstellar civilization they confront ruthless pursuers, ancient and dangerous powers, and hidden conspiracies, forcing him to grow into a reluctant protector and to harness unexpected talents in a blend of swashbuckling space adventure, witty camaraderie, and magical intrigue.