AHSME 1962 Problem 39

Two medians of a triangle with unequal sides are $3$ inches and $6$ inches. Its area is $3 \sqrt{15}$ square inches. The length of the third median in inches, is: $\textbf{(A)}\ 4 \textbf{(B)}\ 3\sqrt{3} \textbf{(C)}\ 3\sqrt{6} \textbf{(D)}\ 6\sqrt{3} \textbf{(E)}\ 6\sqrt{6}$