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

Here,

x + 7 + y + 5 = multiple of 3

x + y + 12 = 0, 3, 6, 9, ….

Hence,

x + y + 12 = 12

∴ x + y = 0

But x + y cannot be 0 because then x and y both will have to be 0.
Since x is the first digit, it cannot be 0.
∴ x + y + 12 = 15

x + y = 3

∴ the least value of (x + y) is 3