\[ a^3 (-y(x))+3 a^2 y'(x)-3 a y''(x)-e^{a x}+y^{(3)}(x)=0 \] ✓ Mathematica : cpu = 0.0165729 (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 = 0.02 (sec), leaf count = 36
\[ \left \{ y \left ( x \right ) ={\frac {{x}^{3}{{\rm e}^{ax}}}{6}}+{\it \_C1}\,{{\rm e}^{ax}}+{\it \_C2}\,{{\rm e}^{ax}}x+{\it \_C3}\,{{\rm e}^{ax}}{x}^{2} \right \} \]