lonic compounds are more soluble in water or in aqueous medium.
lonic character $\propto$ size of cation (if anion is same)
The order of size of cation is
$\mathrm{Na}^{+}>\mathrm{Zn}^{2+}>\mathrm{Cu}^{2+}$
$\therefore$ The order of ionic character and hence, of solubility in water is as
$\mathrm{Na}_2 \mathrm{~S}>\mathrm{ZnS}>\mathrm{CuS}$