Here, our given number is 7x3, which is divisible by 3.

We know that if number is divisible by 3 only when sum of digits of given number is divisible by 3.

Hence, if 7x3 is divisible by 3, then,

7 + x + 3 = 0, 3, 6, 9,12,…

∴ 10 + x = multiple of 3.

Hence, x = (multiple of 3) – 10

Here, possible values for multiples of 3 are 12, 15 and 18

Hence, x = 2, 5 or 8

Hence, possible numbers are 723, 753 and 783