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

Terms

a(0) =1a(1) =3a(2) =3a(3) =7a(4) =13a(5) =23a(6) =43a(7) =79a(8) =145a(9) =267a(10) =491a(11) =903a(12) =1661a(13) =3055a(14) =5619a(15) =10335a(16) =19009a(17) =34963a(18) =64307a(19) =118279a(20) =217549a(21) =400135a(22) =735963a(23) =1353647a(24) =2489745a(25) =4579355a(26) =8422747a(27) =15491847a(28) =28493949a(29) =52408543