\[ \boxed { {\frac {{\rm d}^{3}}{{\rm d}{x}^{3}}}y \left ( x \right ) +2\,f \left ( x \right ) {\frac {\rm d}{{\rm d}x}}y \left ( x \right ) + \left ( {\frac {\rm d}{{\rm d}x}}f \left ( x \right ) \right ) y \left ( x \right ) =0} \]
Mathematica: cpu = 0.072509 (sec), leaf count = 26 \[ \text {DSolve}\left [y(x) f'(x)+2 f(x) y'(x)+y^{(3)}(x)=0,y(x),x\right ] \]
Maple: cpu = 0.063 (sec), leaf count = 24 \[ \left \{ y \left ( x \right ) = \left ( {\it DESol} \left ( \left \{ { \frac {{\rm d}^{2}}{{\rm d}{x}^{2}}}{\it \_Y} \left ( x \right ) +{ \frac {f \left ( x \right ) {\it \_Y} \left ( x \right ) }{2}} \right \} , \left \{ {\it \_Y} \left ( x \right ) \right \} \right ) \right ) ^{2} \right \} \]