I want to make a number 3 balloon sculpture for my daughter's 3rd birthday. I've looked for hours on how to do this and have come up with nothing! I know how to make the balloon quads and all that. But what do I put on the inside of the 3 to shape it?
I am not quite sure I understand what you mean with "what do I put on the inside of the 3 to shape it," but I'll try to answer as best as I can.
Your first step would be to create the frame for your number 3 balloon, using 1/4" or 1/2" aluminium rod, depending on how big and sturdy you want your sculpture to be.
To create the number 3 shape, you could make to half circles, and then tape them together in the middle with duct tape.
After that you'd add your 4 balloon clusters to the frame.
I found this video which shows the principle quite nicely, only that it is about making a letter (E) out of balloons.