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

Terms

a(0) =1a(1) =1a(2) =1a(3) =6a(4) =21a(5) =76a(6) =276a(7) =1001a(8) =3631a(9) =13171a(10) =47776a(11) =173301a(12) =628626a(13) =2280256a(14) =8271321a(15) =30003101a(16) =108832201a(17) =394774126a(18) =1431989881a(19) =5194350096a(20) =18841804176a(21) =68346102601a(22) =247916266251a(23) =899282808131