The standard reduction potential is a measure of the tendency of a chemical species to get reduced. A more negative standard reduction potential indicates a greater tendency to undergo oxidation, which means it acts as a stronger reducing agent. Given standard reduction potentials are: $E^\circ_{\text{X}} = -1.2 \text{ V}$ $E^\circ_{\text{Y}} = +0.5 \text{ V}$ $E^\circ_{\text{Z}} = -3.0 \text{ V}$ As the value of standard reduction potential decreases, the reducing power increases. The order of standard reduction potential is: $E^\circ_{\text{Z}} < E^\circ_{\text{X}} < E^\circ_{\text{Y}}$. Therefore, the order of reducing power of the given metals is: $\text{Z} > \text{X} > \text{Y}$.