\[ \boxed { {\frac {{\rm d}^{3}}{{\rm d}{x}^{3}}}y \left ( x \right ) -3\, \left ( 2\,{\it WeierstrassP} \left ( x,{\it g2},{\it g3} \right ) +a \right ) {\frac {\rm d}{{\rm d}x}}y \left ( x \right ) +by \left ( x \right ) =0} \]
Mathematica: cpu = 0.020003 (sec), leaf count = 30 \[ \text {DSolve}\left [-3 y'(x) (a+2 \wp (x;\text {g2},\text {g3}))+b y(x)+y^{(3)}(x)=0,y(x),x\right ] \]
Maple: cpu = 0.203 (sec), leaf count = 37 \[ \left \{ y \left ( x \right ) ={\it DESol} \left ( \left \{ {\frac { {\rm d}^{3}}{{\rm d}{x}^{3}}}{\it \_Y} \left ( x \right ) + \left ( -6\,{ \it WeierstrassP} \left ( x,{\it g2},{\it g3} \right ) -3\,a \right ) { \frac {\rm d}{{\rm d}x}}{\it \_Y} \left ( x \right ) +b{\it \_Y} \left ( x \right ) \right \} , \left \{ {\it \_Y} \left ( x \right ) \right \} \right ) \right \} \]