✓ Mathematica : cpu = 0.0151017 (sec), leaf count = 46
DSolve[-E^(a*x) - a^3*y[x] + 3*a^2*Derivative[1][y][x] - 3*a*Derivative[2][y][x] + Derivative[3][y][x] == 0,y[x],x]
✓ Maple : cpu = 0.024 (sec), leaf count = 27
dsolve(diff(diff(diff(y(x),x),x),x)-3*a*diff(diff(y(x),x),x)+3*a^2*diff(y(x),x)-a^3*y(x)-exp(a*x)=0,y(x))