We know that the oxidation state of $13$ group elements are the members of the boron family, and so they possess $+3$ oxidation state while $16$ group element are the members of the oxygen family, and they possess $-2$ oxidation state.

So, from above concept we can conclude that the compound formed as ${\mathrm{A}}_2{\mathrm{B}}_3$ as shown below: 

$\begin{array}{ccc}{\mathrm{A}}^{+3}& & {\mathrm{B}}^{2-}\\ & \searrow \swarrow & \\ 3& & 2\end{array}$