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

Terms

a(0) =0a(1) =2a(2) =1a(3) =3a(4) =6a(5) =10a(6) =19a(7) =35a(8) =64a(9) =118a(10) =217a(11) =399a(12) =734a(13) =1350a(14) =2483a(15) =4567a(16) =8400a(17) =15450a(18) =28417a(19) =52267a(20) =96134a(21) =176818a(22) =325219a(23) =598171a(24) =1100208a(25) =2023598a(26) =3721977a(27) =6845783a(28) =12591358a(29) =23159118