\[ y''(x)-\left (2 e^x+1\right ) y'(x)+e^{2 x} y(x)-e^{3 x}=0 \] ✓ Mathematica : cpu = 0.0486449 (sec), leaf count = 28
\[\left \{\left \{y(x)\to c_1 e^{e^x}+c_2 e^{x+e^x}+e^x+2\right \}\right \}\]
✓ Maple : cpu = 0.382 (sec), leaf count = 61
\[ \left \{ y \left ( x \right ) = \left ( {\it \_C1}\,\cosh \left ( {\frac {x}{2}} \right ) +{\it \_C2}\,\sinh \left ( {\frac {x}{2}} \right ) \right ) {{\rm e}^{{{\rm e}^{x}}+{\frac {x}{2}}}}+ \left ( \left ( {{\rm e}^{2\,x}}+{{\rm e}^{x}}+1 \right ) \cosh \left ( {\frac {x}{2}} \right ) -3\, \left ( {{\rm e}^{x}}+1/3\,{{\rm e}^{2\,x}}+1 \right ) \sinh \left ( x/2 \right ) \right ) {{\rm e}^{{\frac {x}{2}}}} \right \} \]