\[ \boxed { \left ( \left ( {\frac {\rm d}{{\rm d}x}}y \left ( x \right ) \right ) ^{2}+1 \right ) {\frac {{\rm d}^{3}}{{\rm d}{x}^{3}}}y \left ( x \right ) -3\, \left ( {\frac {\rm d}{{\rm d}x}}y \left ( x \right ) \right ) \left ( {\frac {{\rm d}^{2}}{{\rm d}{x}^{2}}}y \left ( x \right ) \right ) ^{2}=0} \]
Mathematica: cpu = 0.124016 (sec), leaf count = 95 \[ \left \{\left \{y(x)\to c_3-\frac {i \sqrt {c_1^2 x^2+2 c_2 c_1^2 x+c_2^2 c_1^2-1}}{c_1}\right \},\left \{y(x)\to c_3+\frac {i \sqrt {c_1^2 x^2+2 c_2 c_1^2 x+c_2^2 c_1^2-1}}{c_1}\right \}\right \} \]
Maple: cpu = 0.171 (sec), leaf count = 49 \[ \left \{ y \left ( x \right ) =-\sqrt {-{{\it \_C2}}^{2}-2\,{\it \_C2}\, x-{x}^{2}+{\it \_C1}}+{\it \_C3},y \left ( x \right ) =\sqrt {-{{\it \_C2}}^{2}-2\,{\it \_C2}\,x-{x}^{2}+{\it \_C1}}+{\it \_C3} \right \} \]