Question

write a 3-digit number using digits 2, 9, 4

Answer 1

294, Lol you said it yourself. But there you go c:

Answer 2

9+2+4?

Maybe just put them together to make a 3-digit number; period?

You know what, I'll just do them all!

9+4+2=15

and 

249,942,429,492,924!