a(n) + a(n+1) + a(n+2) = 3^n.

Terms

a(0) =0a(1) =0a(2) =3a(3) =6a(4) =18a(5) =57a(6) =168a(7) =504a(8) =1515a(9) =4542a(10) =13626a(11) =40881a(12) =122640a(13) =367920a(14) =1103763a(15) =3311286a(16) =9933858a(17) =29801577a(18) =89404728a(19) =268214184a(20) =804642555a(21) =2413927662a(22) =7241782986a(23) =21725348961a(24) =65176046880a(25) =195528140640a(26) =586584421923