2005 AMC 10A Problem 2

For each pair of real numbers $a \neq b$, define the operation $\star$ as $(a \star b) = \frac{a+b}{a-b}.$ What is the value of $((1 \star 2) \star 3)$?

$\mathrm{(A) \ } -\frac{2}{3}\qquad \mathrm{(B) \ } -\frac{1}{5} \qquad \mathrm{(C) \ } 0\qquad \mathrm{(D) \ } \frac{1}{2}\qquad \mathrm{(E) \ } \textrm{This value is not defined.}$