Fill in the blank

We know that a number is a multiple of 3 if the sum of all the digits in that number is a multiple of 3.

2 + 0 + x + 3 = x + 5

Since x>0, x + 5’s the closest multiples of 3 are 6,9 and 12.

⇒ x + 5 = 6 gives x = 1

⇒ x + 5 = 9 gives x = 4

⇒ x + 5 = 12 gives x = 7

⇒ Therefore, x is 1, 4 or 7.