a(n) = 3*(6^n - 1)/5.

Terms

a(0) =3a(1) =21a(2) =129a(3) =777a(4) =4665a(5) =27993a(6) =167961a(7) =1007769a(8) =6046617a(9) =36279705a(10) =217678233a(11) =1306069401a(12) =7836416409a(13) =47018498457a(14) =282110990745