How is the oxidation number related to the group number?
For main group elements (groups 1, 2, 13-18), the highest possible oxidation state $O_\text{max}$ is given by the group number $G$ (for groups 1 and 2) or by the group number - 10 (for the rest):
$$ O_\text{max} = \begin{cases} G &amp;\text{if } G \in \{1,2\} \\
G - 10 &amp;\text{if } 13 \leq G \leq 18 \end{cases} $$ 

This means that for your compound $\ce{\stackrel{VI}{X}O3}$, so with the oxidation state VI, the corresponding group would be group 16.