a(n) = a(n-1) + a(n-2) + a(n-3), with a(0) = 1, a(1) = a(2) = 5.

Terms

a(0) =1a(1) =5a(2) =5a(3) =11a(4) =21a(5) =37a(6) =69a(7) =127a(8) =233a(9) =429a(10) =789a(11) =1451a(12) =2669a(13) =4909a(14) =9029a(15) =16607a(16) =30545a(17) =56181a(18) =103333a(19) =190059a(20) =349573a(21) =642965a(22) =1182597a(23) =2175135a(24) =4000697a(25) =7358429a(26) =13534261a(27) =24893387a(28) =45786077a(29) =84213725