Bards, Clerics, and Druids all use the Ritual Magic rules (GURPS Basic Set pg. 242, GURPS Magic pg. 200, GURPS Thaumatology pg. 72).
The core skill for Clerics is Ritual Magic (Clerical Magic); for Druids is Ritual Magic (Druish Magic); and Bards use a Very Hard variant of the Singing skill.
Clerics and Druids use Power Investiture instead of Magery, and Bards use their Bardic Talent level.
All Clerics get the Healing school as a path, plus any additional paths provided by their domain abilities.
Druids may have Animal, Plant, and Weather schools as paths.
Bards may make use of Communication and Empathy, Knowledge, Mind Control, and Sound.