Or do a delayed thing (like a real combo). Where the spells aren't cast simultaneousely, but right after eachother. Have three timers that calls the next one in the series once the previous spell was cast.

Ex. I select fire, ice, and water spells.

Fire - 12 dmg (1 second casting time)
ice - 6 dmg (.5 second casting time)
water - 18 dmg (1.5 second casting time)

Then I hit the combo button:
Fire starts and a special timer is activated, once it hits 1 second (when fire is done), it will start ice automatically, then it will start water once ice time is done.

A simultaneous cast would just be the same thing, except all the timers delay time (spell casting time) would be set to 1 (this would cause each spell to be cast 1 millisecond after the other). It isn't simultaneous, but it is close.

I think maybe he means like an FF type deal. In FF:CC you can merge 3 fira spells to form like Flazaga or something, a more powerful spell. Maybe you meant like that? If so, you can save them to memory. Have a Player.Spell1, 2, 3 or whatever, then check to see what spells are in the slots, and check a like "combo spell list" which would be different from normal spells. it checks the spells slots, and if the slots match a spell in the combo spell list, then cast that combo spell, however if the spells match no combo, then just cast the spell in slot 1. You could design combos for 2 and 3 slots, and then have it so if you have just 1 slot filled, use that slot.

Just an idea, maybe more of what you originally thought?