\[ y^{(3)}(x)-a x^b y(x)=0 \] ✓ Mathematica : cpu = 0.0238331 (sec), leaf count = 164
\[\left \{\left \{y(x)\to (-1)^{\frac {1}{b+3}} (b+3)^{-\frac {6}{b+3}} x a^{\frac {1}{b+3}} \left ((-1)^{\frac {1}{b+3}} c_3 x a^{\frac {1}{b+3}} \, _0F_2\left (;1+\frac {1}{b+3},1+\frac {2}{b+3};\frac {a x^{b+3}}{(b+3)^3}\right )+(b+3)^{\frac {3}{b+3}} c_2 \, _0F_2\left (;1-\frac {1}{b+3},1+\frac {1}{b+3};\frac {a x^{b+3}}{(b+3)^3}\right )\right )+c_1 \, _0F_2\left (;1-\frac {2}{b+3},1-\frac {1}{b+3};\frac {a x^{b+3}}{(b+3)^3}\right )\right \}\right \}\]
✓ Maple : cpu = 0.123 (sec), leaf count = 114
\[ \left \{ y \left ( x \right ) ={\it \_C1}\,{\mbox {$_0$F$_2$}(\ ;\,{\frac {b+1}{b+3}},{\frac {b+2}{b+3}};\,{\frac {{x}^{b+3}a}{ \left ( b+3 \right ) ^{3}}})}+{\it \_C2}\,x{\mbox {$_0$F$_2$}(\ ;\,{\frac {b+2}{b+3}},{\frac {b+4}{b+3}};\,{\frac {{x}^{b+3}a}{ \left ( b+3 \right ) ^{3}}})}+{\it \_C3}\,{x}^{2}{\mbox {$_0$F$_2$}(\ ;\,{\frac {b+5}{b+3}},{\frac {b+4}{b+3}};\,{\frac {{x}^{b+3}a}{ \left ( b+3 \right ) ^{3}}})} \right \} \]