So it's been years since I've played. I know the rulings for regular Polymerization, but for Greater Poly, does it mean you can use any 3 monsters, or does it have to be the exact materials listed on the fusion monster card?
Compare the two effect texts. From Polymerization (emphasis added):
Fusion Summon 1 Fusion Monster from your Extra Deck, using monsters from your hand or field as Fusion Material.
And from Greater Polymerization:
Fusion Summon 1 Fusion Monster from your Extra Deck, using 3 or more monsters from your hand or field as Fusion Material, and if you do, it gains these effects.
Then it lists its effects. In other words, they must be Fusion Material as required by the fusion monster. The benefit is that it provides its two effects: invincible to card effects and piercing damage.