y=var('y')
y=function('y',x)
dy=diff(y,x,3)+9*diff(y,x,2)+27*diff(y,x,1)+27*y==0
desolve_laplace(dy,y,x)

sol:
        

9/2*x^2*e^(-3*x)*y(0) + 3*x^2*e^(-3*x)*D[0](y)(0) +
1/2*x^2*e^(-3*x)*D[0, 0](y)(0) + 1/2*(3*y(0) + D[0](y)(0))*x*e^(-3*x) +
3/2*x*e^(-3*x)*y(0) + 1/2*x*e^(-3*x)*D[0](y)(0) + e^(-3*x)*y(0)