a(1)=1; a(2)=2; a(3)=3; a(n) = Sum_{k=3..n-1} (a(k) + a(k-1) + a(k-2)).

Terms

a(0) =1a(1) =2a(2) =3a(3) =6a(4) =17a(5) =43a(6) =109a(7) =278a(8) =708a(9) =1803a(10) =4592a(11) =11695a(12) =29785a(13) =75857a(14) =193194a(15) =492030a(16) =1253111a(17) =3191446a(18) =8128033a(19) =20700623a(20) =52720725a(21) =134270106a(22) =341961560a(23) =870913951a(24) =2218059568a(25) =5648994647a(26) =14386962813a(27) =36640979841a(28) =93317917142