a(n) = 3^n*tribonacci(n) or (3^n)*A001644(n+1).

Terms

a(0) =3a(1) =27a(2) =189a(3) =891a(4) =5103a(5) =28431a(6) =155277a(7) =859491a(8) =4743603a(9) =26158707a(10) =144374805a(11) =796630059a(12) =4395548511a(13) =24254435799a(14) =133832255589a(15) =738466498755