Aion Online. Spell (and skill) combo system that works. Basically you start with a 'basic' attack that triggers a secondary or tertiary attack depending on the class and you have a few seconds to choose which one to do. This way, you have your '3 hit combo' with a big skill at the end. You could experiment with an RNG chance of the 3rd skill triggering (and thus making them more powerful) but I'd prefer you avoid that RNG route. It is easy/cheap/lazy and it invites frustration rather than reward. The player won due to luck rather than skill, basically.
Personally as a player, I feel that having a combo system would be nice to have. Set up custom combos that roll into each other (TERA did a custom combo system really well) smoothly. Basically classify everything as Level 1, Level 2, and Level 3. Which 'starter' Level 1 card would determine which 'path' you follow for Level 2; in turn which Level 2 skill you choose determines which Level 3 card you end up using/casting.
Example? L1 Fireball into L2 Ice Lance into L3 Toxic Bomb. Different elements but really flexible for the situation.