Tenemos la indeterminación de tipo
0/0,
tal que el límite para el numerador es
$$\lim_{x \to 0^+} \operatorname{atan}{\left(3 x \right)} = 0$$
y el límite para el denominador es
$$\lim_{x \to 0^+}\left(- \frac{1}{\cot{\left(x \right)}}\right) = 0$$
Vamos a probar las derivadas del numerador y denominador hasta eliminar la indeterminación.
$$\lim_{x \to 0^+}\left(\cot{\left(x \right)} \left(- \operatorname{atan}{\left(3 x \right)}\right)\right)$$
=
Introducimos una pequeña modificación de la función bajo el signo del límite
$$\lim_{x \to 0^+}\left(- \cot{\left(x \right)} \operatorname{atan}{\left(3 x \right)}\right)$$
=
$$\lim_{x \to 0^+}\left(\frac{\frac{d}{d x} \operatorname{atan}{\left(3 x \right)}}{\frac{d}{d x} \left(- \frac{1}{\cot{\left(x \right)}}\right)}\right)$$
=
$$\lim_{x \to 0^+}\left(- \frac{3 \cot^{2}{\left(x \right)}}{\left(9 x^{2} + 1\right) \left(\cot^{2}{\left(x \right)} + 1\right)}\right)$$
=
$$\lim_{x \to 0^+}\left(- \frac{3 \cot^{2}{\left(x \right)}}{\cot^{2}{\left(x \right)} + 1}\right)$$
=
$$\lim_{x \to 0^+}\left(\frac{\frac{d}{d x} \frac{1}{\cot^{2}{\left(x \right)} + 1}}{\frac{d}{d x} \left(- \frac{1}{3 \cot^{2}{\left(x \right)}}\right)}\right)$$
=
$$\lim_{x \to 0^+}\left(\frac{1}{\left(2 \cot^{2}{\left(x \right)} + 2\right) \left(\frac{\cot^{4}{\left(x \right)}}{- 6 \cot^{6}{\left(x \right)} - 6 \cot^{4}{\left(x \right)}} + \frac{2 \cot^{2}{\left(x \right)}}{- 6 \cot^{6}{\left(x \right)} - 6 \cot^{4}{\left(x \right)}} + \frac{1}{- 6 \cot^{6}{\left(x \right)} - 6 \cot^{4}{\left(x \right)}}\right)}\right)$$
=
$$\lim_{x \to 0^+}\left(\frac{\frac{d}{d x} \frac{1}{2 \cot^{2}{\left(x \right)} + 2}}{\frac{d}{d x} \left(\frac{\cot^{4}{\left(x \right)}}{- 6 \cot^{6}{\left(x \right)} - 6 \cot^{4}{\left(x \right)}} + \frac{2 \cot^{2}{\left(x \right)}}{- 6 \cot^{6}{\left(x \right)} - 6 \cot^{4}{\left(x \right)}} + \frac{1}{- 6 \cot^{6}{\left(x \right)} - 6 \cot^{4}{\left(x \right)}}\right)}\right)$$
=
$$\lim_{x \to 0^+}\left(- \frac{2 \left(- 2 \cot^{2}{\left(x \right)} - 2\right) \cot{\left(x \right)}}{\left(2 \cot^{2}{\left(x \right)} + 2\right)^{2} \left(\frac{\left(6 \left(- 6 \cot^{2}{\left(x \right)} - 6\right) \cot^{5}{\left(x \right)} + 6 \left(- 4 \cot^{2}{\left(x \right)} - 4\right) \cot^{3}{\left(x \right)}\right) \cot^{4}{\left(x \right)}}{\left(- 6 \cot^{6}{\left(x \right)} - 6 \cot^{4}{\left(x \right)}\right)^{2}} + \frac{2 \left(6 \left(- 6 \cot^{2}{\left(x \right)} - 6\right) \cot^{5}{\left(x \right)} + 6 \left(- 4 \cot^{2}{\left(x \right)} - 4\right) \cot^{3}{\left(x \right)}\right) \cot^{2}{\left(x \right)}}{\left(- 6 \cot^{6}{\left(x \right)} - 6 \cot^{4}{\left(x \right)}\right)^{2}} + \frac{6 \left(- 6 \cot^{2}{\left(x \right)} - 6\right) \cot^{5}{\left(x \right)} + 6 \left(- 4 \cot^{2}{\left(x \right)} - 4\right) \cot^{3}{\left(x \right)}}{\left(- 6 \cot^{6}{\left(x \right)} - 6 \cot^{4}{\left(x \right)}\right)^{2}} + \frac{\left(- 4 \cot^{2}{\left(x \right)} - 4\right) \cot^{3}{\left(x \right)}}{- 6 \cot^{6}{\left(x \right)} - 6 \cot^{4}{\left(x \right)}} + \frac{2 \left(- 2 \cot^{2}{\left(x \right)} - 2\right) \cot{\left(x \right)}}{- 6 \cot^{6}{\left(x \right)} - 6 \cot^{4}{\left(x \right)}}\right)}\right)$$
=
$$\lim_{x \to 0^+}\left(- \frac{2 \left(- 2 \cot^{2}{\left(x \right)} - 2\right) \cot{\left(x \right)}}{\left(2 \cot^{2}{\left(x \right)} + 2\right)^{2} \left(\frac{\left(6 \left(- 6 \cot^{2}{\left(x \right)} - 6\right) \cot^{5}{\left(x \right)} + 6 \left(- 4 \cot^{2}{\left(x \right)} - 4\right) \cot^{3}{\left(x \right)}\right) \cot^{4}{\left(x \right)}}{\left(- 6 \cot^{6}{\left(x \right)} - 6 \cot^{4}{\left(x \right)}\right)^{2}} + \frac{2 \left(6 \left(- 6 \cot^{2}{\left(x \right)} - 6\right) \cot^{5}{\left(x \right)} + 6 \left(- 4 \cot^{2}{\left(x \right)} - 4\right) \cot^{3}{\left(x \right)}\right) \cot^{2}{\left(x \right)}}{\left(- 6 \cot^{6}{\left(x \right)} - 6 \cot^{4}{\left(x \right)}\right)^{2}} + \frac{6 \left(- 6 \cot^{2}{\left(x \right)} - 6\right) \cot^{5}{\left(x \right)} + 6 \left(- 4 \cot^{2}{\left(x \right)} - 4\right) \cot^{3}{\left(x \right)}}{\left(- 6 \cot^{6}{\left(x \right)} - 6 \cot^{4}{\left(x \right)}\right)^{2}} + \frac{\left(- 4 \cot^{2}{\left(x \right)} - 4\right) \cot^{3}{\left(x \right)}}{- 6 \cot^{6}{\left(x \right)} - 6 \cot^{4}{\left(x \right)}} + \frac{2 \left(- 2 \cot^{2}{\left(x \right)} - 2\right) \cot{\left(x \right)}}{- 6 \cot^{6}{\left(x \right)} - 6 \cot^{4}{\left(x \right)}}\right)}\right)$$
=
$$-3$$
Como puedes ver, hemos aplicado el método de l'Hopital (utilizando la derivada del numerador y denominador) 3 vez (veces)