Rune Spell System Question

I had an idea for a spell system. It wouldn't be the typical select a spell from a list kind of system. When you want to cast a spell 3 slots appear. Then you choose a sequence of spells from a list of runes that you have.

Example:

Battle with a goblin begins.
You want to cast a fireball spell.
3 slots appear
You select Rune A, then Rune B, then Rune C.
If that is the right combination for the Fireball spell, then Fireball is cast at the enemy
If the combination is wrong then the spell fizzles and does nothing or casts a different spell all together.

Got any ideas?

I think I have a way of doing it without a script. But it would be like Oblivion where you go to a pedestal or something and combine 3 runes to make a spell that you cast from a list.