Irrational and rational proofs

Hey, I was wondering about proving that square roots of numbers are rational or irrational.
I know about proving this for the numbers 1(duh), 2 and all multiples of 2 and 4, but what about 3?
How do i prove that 3(or any other number) is irrational or rational?

So why not just call it always, the futher into infinty you go, the smaller the odds get, so the odds are infinatly small. you might as well call the odds 0