a(n) = 3*a(n-1) + 5*a(n-2) + a(n-3).

Terms

a(0) =1a(1) =6a(2) =23a(3) =100a(4) =421a(5) =1786a(6) =7563a(7) =32040a(8) =135721a(9) =574926a(10) =2435423a(11) =10316620a(12) =43701901a(13) =185124226a(14) =784198803a(15) =3321919440a(16) =14071876561a(17) =59609425686a(18) =252509579303