An ionic compound has a unit cell consisting of $A$ ions at the corners of a cube and $B$ ions on the centres of the faces of the cube. The empirical formula for this compound would be:

Number of $A$ ions: $A$ ions are at the $8$ corners. Each corner atom contributes $1/8$ to the unit cell.
$$\text{Total }A=8\times \frac{1}{8}=1$$
Number of $B$ ions: $B$ ions are at the $6$ face centers. Each face center atom contributes $1/2$ to the unit cell.
$$\text{Total }B=6\times \frac{1}{2}=3$$
The ratio of $A:B$ is $1:3$. Thus, the empirical formula is ${\text{AB}}_3$.