$\cot \dfrac{A}2 + \cot \dfrac{B}2 + \cot \dfrac{C}2 = \dfrac{\sin ( \dfrac{A}2 + \dfrac{B}2)}{\sin \dfrac{A}2 \cdot \sin \dfrac{B}2} + \cot \dfrac{C}2$
$= \dfrac{\cos \dfrac{C}2}{\sin \dfrac{A}2 \cdot \sin \dfrac{B}2} + \cot \dfrac{C}2$
$= \cot \dfrac{C}2 \cdot \left( \dfrac{\sin \dfrac{C}2}{\sin \dfrac{A}2 \cdot \sin \dfrac{B}2} + 1 \right)$
$= \cot \dfrac{C}2 \cdot \dfrac{\cos (\dfrac{A}2 + \dfrac{B}2) + \sin \dfrac{A}2 \cdot \sin \dfrac{B}2}{\sin \dfrac{A}2 \cdot \sin \dfrac{B}2}$
$= \cot \dfrac{C}2 \cdot \dfrac{\cos \dfrac{A}2 \cdot \cos \dfrac{B}2}{\sin \dfrac{A}2 \cdot \sin \dfrac{B}2}$
$= \cot \dfrac{C}2 \cdot \cot \dfrac{A}2 \cdot \cot \dfrac{B}2$