I wonder if it would be easier to make them from a 12x12 sheet? I'm terrible with math, so that is the way I would go... I think you can get 8 3x3 cards from a 12x12 sheet with no waste.
I did a Google search for 3x3 cards, and lots of images came up:
If you are on Pinterest, you could search there for ideas, too...
Good luck! I would love to see what you come up with!
The Crafty Sister
Designing for: YourNextStamp, C.C.Designs, WhimsieDoodles, LittleMissMuffet, PaperieInk