Step 1: Part (I): Fe or Cu - which has the higher melting point?
Iron (Fe) has the higher melting point. Fe: m.p. $= 1538^\circ C$; Cu: m.p. $= 1085^\circ C$. Reason: Fe ($[Ar]3d^64s^2$) has more unpaired $d$-electrons that contribute to metallic bonding. Strong metallic bonding (from both $3d$ and $4s$ electrons) gives Fe a high melting point. Cu ($[Ar]3d^{10}4s^1$) has a completely filled $3d^{10}$ shell; the $3d$ electrons do not contribute effectively to metallic bonding, so Cu has weaker metallic bonds and a lower melting point.
Step 2: Part (II): $Ti^{3+}$ or $Sc^{3+}$ - which is coloured in aqueous solution?
$Ti^{3+}$ is coloured (violet in aqueous solution); $Sc^{3+}$ is colourless. Reason: Colour in transition metal ions arises from $d$-$d$ electronic transitions. $Sc^{3+}$: configuration $[Ar]3d^0$ - no $d$-electrons, no $d$-$d$ transition possible, so no visible light absorbed, colourless. $Ti^{3+}$: configuration $[Ar]3d^1$ - one $d$-electron can absorb visible light and jump to a higher $d$-orbital ($d$-$d$ transition), so $Ti^{3+}$ appears coloured (violet).
Step 3: Part (III): Cr or Zn - which has the higher third ionisation enthalpy?
Zinc (Zn) has the higher third ionisation enthalpy than chromium (Cr). Configuration: $Zn^{2+}$: $[Ar]3d^{10}$ (completely filled $3d$). Removing a 3rd electron from $Zn^{2+}$ requires breaking into the stable, completely filled $3d^{10}$ configuration, which needs very high energy. $Cr^{2+}$: $[Ar]3d^4$, less stable than $3d^{10}$; removing a 3rd electron requires less energy. So Zn has higher 3rd ionisation enthalpy.
Step 4: Explain why filled $3d^{10}$ leads to high ionisation enthalpy.
The completely filled $3d^{10}$ subshell of $Zn^{2+}$ is specially stable (exchange energy is maximised, no distortion). Removing any electron from this stable configuration requires overcoming significant stability, so the 3rd ionisation enthalpy is very high.
Step 5: General trend in ionisation enthalpies across d-block.
Ionisation enthalpies generally increase from left to right across the $d$-block as nuclear charge increases. However, anomalies occur at half-filled ($d^5$) and fully-filled ($d^{10}$) configurations, where extra stability leads to higher-than-expected ionisation enthalpies.
Step 6: Summarise all three answers.
(I) Fe: higher melting point (stronger metallic bonding from $3d$ + $4s$ electrons). (II) $Ti^{3+}$: coloured in solution ($3d^1$ enables $d$-$d$ transitions); $Sc^{3+}$ is colourless ($3d^0$). (III) Zn: higher 3rd ionisation enthalpy (breaking stable $3d^{10}$ of $Zn^{2+}$). \[ \boxed{(I)\;Fe;\;(II)\;Ti^{3+}\;(3d^1\text{ coloured});\;(III)\;Zn\;(\text{stable }3d^{10})} \]