The Sorceress Of Karres by Eric Flint

A mild-mannered academic and small-time trader finds his life upended when three mischievous young girls from a remote planet enlist his help and reveal startling sorcerous abilities; drawn into a witty, fast-paced adventure across the space lanes, he must outwit pirates, shadowy agents, and strange alien forces while trying to keep his unexpected charges—and himself—alive. The story blends swashbuckling space opera, clever humor, and elements of mystery as the unlikely guardian uncovers the true nature and dangerous consequences of the witches' powers.