For each pair of real numbers $a \neq b$, define the [operation](https://artofproblemsolving.com/wiki/index.php/Operation) $\star$ as
$(a \star b) = \frac{a+b}{a-b}$.
What is the value of $((1 \star 2) \star 3)$?
$\textbf{(A) } -\frac{2}{3}\qquad \textbf{(B) } -\frac{1}{5}\qquad \textbf{(C) } 0\qquad \textbf{(D) } \frac{1}{2}\qquad \textbf{(E) } \textrm{This\, value\, is\, not\, defined.}$
$((1 \star 2) \star 3) = \left(\left(\frac{1+2}{1-2}\right) \star 3\right) = (-3 \star 3) = \frac{-3+3}{-3-3} = 0 \Longrightarrow \boxed{\textbf{(C) }0}$