We know that if sum of digits of a number is divisible by 3, then the number is divisible by 3.

Here,

4 + x + y + 7 = multiple of 3

x + y + 11 = 0, 3, 6, 9, ….

Hence,

x + y + 11 = 12

∴ x + y = 1

∴ the least value of (x + y) is 1