At stage 3 advancement your skills do not change but your class name does get changed.

Completing Stage 3 Advancement also unlocks the Manastones feature. This feature allows you to collect, use and upgrade Manastones to enhance your own stats. Manastones can be earned through events, Apex lucky wheel and can be purchased at the Colosseum Shop found at the Trading Post.

Mana Stones can be equipped on your skills. To enhance the stones click Mana Stone Enhancement and you can upgrade every Mana Stone by consuming unwanted Mana Stones.

This guide is sourced from yabusame.