a(n) = Sum_{k=0..n} binomial(3*n+k-1,n-k).

Terms

a(0) =1a(1) =3a(2) =17a(3) =103a(4) =643a(5) =4082a(6) =26193a(7) =169324a(8) =1100561a(9) =7183179a(10) =47037770a(11) =308839996a(12) =2032250927a(13) =13397582173a(14) =88463267588a(15) =584916078291