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

Terms

a(0) =0a(1) =1a(2) =3a(3) =11a(4) =40a(5) =145a(6) =526a(7) =1908a(8) =6921a(9) =25105a(10) =91065a(11) =330326a(12) =1198213a(13) =4346356a(14) =15765820a(15) =57188385a(16) =207443151a(17) =752472043a(18) =2729490816a(19) =9900859685a(20) =35914032730a(21) =130273308376a(22) =472548850273