In an HCP lattice, if there are $n$ atoms of B, the number of tetrahedral voids is $2n$. In the formula $A_2B_3$, the ratio of $A:B$ is $2:3$, which can be written as $\frac{2}{3}:1$. If B forms the lattice ($n$ atoms), then A occupies $2/3$ atoms. The fraction of tetrahedral voids occupied by A is $\frac{\text{atoms of A}}{\text{total tetrahedral voids}} = \frac{2/3}{2} = \frac{1}{3}$.