AHSME 1962 Problem 12

When $\left ( 1 - \frac{1}{a} \right ) ^6$ is expanded the sum of the last three coefficients is: $\textbf{(A)}\ 22 \textbf{(B)}\ 11 \textbf{(C)}\ 10 \textbf{(D)}\ -10 \textbf{(E)}\ -11$