\[ \boxed { 2\,x \left ( x-1 \right ) {\frac {{\rm d}^{3}}{{\rm d}{x}^{3}}}y \left ( x \right ) +3\, \left ( 2\,x-1 \right ) {\frac {{\rm d}^{2}}{{\rm d}{x}^{2}}}y \left ( x \right ) + \left ( 2\,ax+b \right ) {\frac {\rm d}{{\rm d}x}}y \left ( x \right ) +ay \left ( x \right ) =0} \]
Mathematica: cpu = 62.084884 (sec), leaf count = 69 \[ \left \{\left \{y(x)\to \text {DifferentialRoot}\left (\{\unicode {f818},\unicode {f817}\}\unicode {f4a1}\left \{a \unicode {f818}(\unicode {f817})+(2 \unicode {f817} a+b) \unicode {f818}'(\unicode {f817})+(6 \unicode {f817}-3) \unicode {f818}''(\unicode {f817})+2 (\unicode {f817}-1) \unicode {f817} \unicode {f818}^{(3)}(\unicode {f817})=0,\unicode {f818}(2)=c_1,\unicode {f818}'(2)=c_2,\unicode {f818}''(2)=c_3\right \}\right )(x)\right \}\right \} \]
Maple: cpu = 0.125 (sec), leaf count = 79 \[ \left \{ y \left ( x \right ) ={\it \_C1}\, \left ( {\it MathieuC} \left ( -{\frac {a}{2}}-{\frac {b}{2}}+1,{\frac {a}{4}},\arccos \left ( \sqrt {x} \right ) \right ) \right ) ^{2}+{\it \_C2}\, \left ( { \it MathieuS} \left ( -{\frac {a}{2}}-{\frac {b}{2}}+1,{\frac {a}{4}}, \arccos \left ( \sqrt {x} \right ) \right ) \right ) ^{2}+{\it \_C3}\,{ \it MathieuC} \left ( -{\frac {a}{2}}-{\frac {b}{2}}+1,{\frac {a}{4}}, \arccos \left ( \sqrt {x} \right ) \right ) {\it MathieuS} \left ( -{ \frac {a}{2}}-{\frac {b}{2}}+1,{\frac {a}{4}},\arccos \left ( \sqrt {x} \right ) \right ) \right \} \]