✓ Mathematica : cpu = 0.0097849 (sec), leaf count = 38
DSolve[18*E^x - 3*y[x] - 11*Derivative[1][y][x] - 8*Derivative[2][y][x] + 4*Derivative[3][y][x] == 0,y[x],x]
✓ Maple : cpu = 0.025 (sec), leaf count = 23
dsolve(4*diff(diff(diff(y(x),x),x),x)-8*diff(diff(y(x),x),x)-11*diff(y(x),x)-3*y(x)+18*exp(x)=0,y(x))