a(n) = a(n-1) + a(n-2) + a(n-3), with a(0) = 1, a(1) = a(2) = 6.

Terms

a(0) =1a(1) =6a(2) =6a(3) =13a(4) =25a(5) =44a(6) =82a(7) =151a(8) =277a(9) =510a(10) =938a(11) =1725a(12) =3173a(13) =5836a(14) =10734a(15) =19743a(16) =36313a(17) =66790a(18) =122846a(19) =225949a(20) =415585a(21) =764380a(22) =1405914a(23) =2585879a(24) =4756173a(25) =8747966a(26) =16090018a(27) =29594157a(28) =54432141a(29) =100116316