\[ \boxed { x{\frac {{\rm d}^{3}}{{\rm d}{x}^{3}}}y \left ( x \right ) +3\,{\frac {{\rm d}^{2}}{{\rm d}{x}^{2}}}y \left ( x \right ) -a{x}^{2}y \left ( x \right ) =0} \]
Mathematica: cpu = 0.032004 (sec), leaf count = 104 \[ \left \{\left \{y(x)\to -\frac {2 (-1)^{3/4} \sqrt {2} c_1 \, _0F_2\left (;\frac {1}{2},\frac {3}{4};\frac {a x^4}{64}\right )}{\sqrt [4]{a} x}+c_2 \, _0F_2\left (;\frac {3}{4},\frac {5}{4};\frac {a x^4}{64}\right )+\frac {\sqrt [4]{-1} \sqrt [4]{a} c_3 x \, _0F_2\left (;\frac {5}{4},\frac {3}{2};\frac {a x^4}{64}\right )}{2 \sqrt {2}}\right \}\right \} \]
Maple: cpu = 0.094 (sec), leaf count = 48 \[ \left \{ y \left ( x \right ) ={\it \_C1}\, {\mbox {$_0$F$_2$}(\ ;\,{\frac {3}{4}},{\frac {5}{4}};\,{\frac {a{x}^{4}}{64}})} +{\frac {{\it \_C2}}{x} {\mbox {$_0$F$_2$}(\ ;\,{\frac {1}{2}},{\frac {3}{4}};\,{\frac {a{x}^{4}}{64}})} }+{\it \_C3}\,x {\mbox {$_0$F$_2$}(\ ;\,{\frac {5}{4}},{\frac {3}{2}};\,{\frac {a{x}^{4}}{64}})} \right \} \]