a(n) = binomial(3n,n).

Terms

a(0) =1a(1) =3a(2) =15a(3) =84a(4) =495a(5) =3003a(6) =18564a(7) =116280a(8) =735471a(9) =4686825a(10) =30045015a(11) =193536720a(12) =1251677700a(13) =8122425444a(14) =52860229080a(15) =344867425584