a(n) = a(n-1) + a(n-2) + a(n-3), with a(0) = 1, a(1) = a(2) = 9.

Terms

a(0) =1a(1) =9a(2) =9a(3) =19a(4) =37a(5) =65a(6) =121a(7) =223a(8) =409a(9) =753a(10) =1385a(11) =2547a(12) =4685a(13) =8617a(14) =15849a(15) =29151a(16) =53617a(17) =98617a(18) =181385a(19) =333619a(20) =613621a(21) =1128625a(22) =2075865a(23) =3818111a(24) =7022601a(25) =12916577a(26) =23757289a(27) =43696467a(28) =80370333a(29) =147824089