Markov Chains & Long-Run Behavior · Easy problem

The third column of a $3 \times 3$ transition matrix (the probabilities of moving out of state 3 into states 1, 2, 3) is $\begin{pmatrix} 1/6 \\ a \\ 1/2 \end{pmatrix}$. What must $a$ be? Express your answer as a common fraction.