Problem 6 (1992 AJHSME)

Suppose that a table with $a$ and $b$ in the first row and $c$ in the second row means $a+b-c$. For example, a table with $5$ and $4$ in the first row and $6$ in the second row is $5+4-6=3$. Then the value of a table with $1$ and $2$ in the first row and $3$ in the second row is: