Standard electrode potential of three metals X, Y and Z are -1.2 V,...

Standard electrode potential generally refers to the standard reduction potential.

A lower (more negative) standard reduction potential indicates a greater tendency for the metal to get oxidized, which means it acts as a stronger reducing agent.

The given standard reduction potentials are:
$E^{{}^{\circ}}_{X} = -1.2$ V
$E^{{}^{\circ}}_{Y} = +0.5$ V
$E^{{}^{\circ}}_{Z} = -3.0$ V

Comparing the values, we have $-3.0 \text{ V} < -1.2 \text{ V} < +0.5 \text{ V}$.

Since $Z$ has the most negative standard reduction potential, it is the strongest reducing agent, followed by $X$ and then $Y$.

Therefore, the order of reducing power is $Z > X > Y$.

Answer: $Z > X > Y$