The Castoffs Series by M.K. Reed, Brian Smith

Twenty years ago, mages fought for their survival against the Surrogate, a hive-minded robot army that consumed everything in its path. Now, a trio of young apprentice mages must cross the ruined forests to deliver a magical cure to a nearby town. But the girls couldn't be more different. Charris, a hotheaded fighter with elemental powers, Trinh, a shy mage who can literally turn invisible, and Ursa, a healer with more power than she lets on... moreTwenty years ago, mages fought for their survival against the Surrogate, a hive-minded robot army that consumed everything in its path. Now, a trio of young apprentice mages must cross the ruined forests to deliver a magical cure to a nearby town. But the girls couldn't be more different. Charris, a hotheaded fighter with elemental powers, Trinh, a shy mage who can literally turn invisible, and Ursa, a healer with more power than she lets on. The three girls must learn to trust and rely on each other if they're going to save the village! less