In $\mathrm{VO}_2^{+}, \mathrm{V}$ is in +3 oxidation state.
$\mathrm{Cr}_2 \mathrm{O}_7^{2-} \mathrm{Cr}$ is in +6 oxidation state.
$\mathrm{MnO}_4^{-} \mathrm{Mn}$ is in +7 oxidation state.
Oxidising power means substance have more tendency to accept electron. Mn is in highest oxidation state so it have more oxidising power.
$\mathrm{MnO}_4^{-}\gt\mathrm{Cr}_2 \mathrm{O}_7^{2-}\gt\mathrm{VO}_2^{+}$