\[ \boxed { {\frac {{\rm d}^{3}}{{\rm d}{x}^{3}}}y \left ( x \right ) -{x}^{2}{\frac {{\rm d}^{2}}{{\rm d}{x}^{2}}}y \left ( x \right ) + \left ( a+b-1 \right ) x{\frac {\rm d}{{\rm d}x}}y \left ( x \right ) -by \left ( x \right ) a=0} \]
Mathematica: cpu = 0.028004 (sec), leaf count = 127 \[ \left \{\left \{y(x)\to \sqrt [3]{-\frac {1}{3}} c_2 x \, _2F_2\left (\frac {1}{3}-\frac {a}{3},\frac {1}{3}-\frac {b}{3};\frac {2}{3},\frac {4}{3};\frac {x^3}{3}\right )+c_1 \, _2F_2\left (-\frac {a}{3},-\frac {b}{3};\frac {1}{3},\frac {2}{3};\frac {x^3}{3}\right )+\left (-\frac {1}{3}\right )^{2/3} c_3 x^2 \, _2F_2\left (\frac {2}{3}-\frac {a}{3},\frac {2}{3}-\frac {b}{3};\frac {4}{3},\frac {5}{3};\frac {x^3}{3}\right )\right \}\right \} \]
Maple: cpu = 0.110 (sec), leaf count = 71 \[ \left \{ y \left ( x \right ) ={\it \_C1}\, {\mbox {$_2$F$_2$}(-{\frac {a}{3}},-{\frac {b}{3}};\,{\frac {1}{3}},{\frac {2}{3}};\,{\frac {{x}^{3}}{3}})} +{\it \_C2}\,x {\mbox {$_2$F$_2$}({\frac {1}{3}}-{\frac {a}{3}},{\frac {1}{3}}-{\frac {b}{3}};\,{\frac {2}{3}},{\frac {4}{3}};\,{\frac {{x}^{3}}{3}})} +{\it \_C3}\,{x}^{2} {\mbox {$_2$F$_2$}(-{\frac {a}{3}}+{\frac {2}{3}},-{\frac {b}{3}}+{\frac {2}{3}};\,{\frac {4}{3}},{\frac {5}{3}};\,{\frac {{x}^{3}}{3}})} \right \} \]