To cast three magic spells in one round, first equip someone with a Magic Materia + "All" Support Materia. You'll also need Barrier Materia with the Reflect spell learned. When you enter battle, cast Reflect on the entire party, (your party). Then cast the magic spell that was connected to the "All" Materia on YOUR party.

Even though you cast the spell once, it will be reflected and returned to the enemy hitting for three times! And for the same price of the usual MP's. What's even better is that if you have W-Magic, you can cast that spell twice, or two different spells (both need to be connected to "All" Materia) hitting for six times!!!