a(n) = a(n-1) + a(n-2) + a(n-3), with a(0) = 1, a(1) = a(2) = 4.

Terms

a(0) =1a(1) =4a(2) =4a(3) =9a(4) =17a(5) =30a(6) =56a(7) =103a(8) =189a(9) =348a(10) =640a(11) =1177a(12) =2165a(13) =3982a(14) =7324a(15) =13471a(16) =24777a(17) =45572a(18) =83820a(19) =154169a(20) =283561a(21) =521550a(22) =959280a(23) =1764391a(24) =3245221a(25) =5968892a(26) =10978504a(27) =20192617a(28) =37140013a(29) =68311134