f(n+3)x^n+3=x^2f(n+1)x^n+1+x^3f(n)x^n
f-x^2f2-xf1-f0=x^2(f-f0)+x^3f
f=(x^2f2+xf1+f0-x^2f0)/(1-x^2)
=(3x^2+2x)/(1-x^2)