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

Terms

a(0) =1a(1) =1a(2) =3a(3) =9a(4) =25a(5) =69a(6) =191a(7) =529a(8) =1465a(9) =4057a(10) =11235a(11) =31113a(12) =86161a(13) =238605a(14) =660767a(15) =1829857a(16) =5067409a(17) =14033137a(18) =38861859a(19) =107619849a(20) =298030825a(21) =825334485a(22) =2285592479a(23) =6329473777a(24) =17528163337a(25) =48540608713a(26) =134423136579