( 1)

A+B-C=3

(2)

A-B-C= 1

(3)

(1)-(2) that is 2C=2.

C= 1。

( 1)+(2)

Namely: (A+B)=4

(4)

Substituting C= 1 into (3) gives: (A-B)=2.

(5)

From (4) and (5), A=3, B= 1,

So the solution of this problem is A=3, B= 1, C= 1.