Here, in units place,

A + 7 = 3

Ie, A = -4, which is not possible.

Hence A is greater than 10,where, 1 carry is given to tens place.

∴A + 7 = 13

∴ A = 6

Now in tens place,

5 + 8 + 1 = B …as 1 is carried

∴ B = 14

Here, 1 will be carried to hundreds place.

Hence, B = 4 and C = 1