Find the missing number?

6
2
9
3

The answer is 3.

The missing number is the number that, when added to the other two numbers, gives a multiple of 9.

In this case, 2 and 6 are both multiples of 3, so the missing number must also be a multiple of 3. The only answer choice that is a multiple of 3 is 3.

Therefore, the missing number is 3.