a(1)=3, a(2)=3, a(n)=3*a(n-1) + 3*a(n-2).

Terms

a(0) =3a(1) =3a(2) =18a(3) =63a(4) =243a(5) =918a(6) =3483a(7) =13203a(8) =50058a(9) =189783a(10) =719523a(11) =2727918a(12) =10342323a(13) =39210723a(14) =148659138a(15) =563609583a(16) =2136806163a(17) =8101247238a(18) =30714160203a(19) =116446222323a(20) =441481147578