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

Terms

a(0) =-1a(1) =2a(2) =2a(3) =3a(4) =7a(5) =12a(6) =22a(7) =41a(8) =75a(9) =138a(10) =254a(11) =467a(12) =859a(13) =1580a(14) =2906a(15) =5345a(16) =9831a(17) =18082a(18) =33258a(19) =61171a(20) =112511a(21) =206940a(22) =380622a(23) =700073a(24) =1287635a(25) =2368330a(26) =4356038a(27) =8012003a(28) =14736371a(29) =27104412