\[ a^3 (-y(x))+3 a^2 y'(x)-3 a y''(x)-e^{a x}+y^{(3)}(x)=0 \] ✓ Mathematica : cpu = 0.0083792 (sec), leaf count = 46
\[\left \{\left \{y(x)\to c_3 x^2 e^{a x}+c_2 x e^{a x}+c_1 e^{a x}+\frac {1}{6} x^3 e^{a x}\right \}\right \}\] ✓ Maple : cpu = 0.011 (sec), leaf count = 27
\[ \left \{ y \left ( x \right ) ={\frac {{{\rm e}^{ax}} \left ( 6\,{\it \_C3}\,{x}^{2}+{x}^{3}+6\,{\it \_C2}\,x+6\,{\it \_C1} \right ) }{6}} \right \} \]