a(n) = 3*a(n-1) - a(n-3), with a(0) = 3, a(1) = 3, and a(2) = 9.

Terms

a(0) =3a(1) =3a(2) =9a(3) =24a(4) =69a(5) =198a(6) =570a(7) =1641a(8) =4725a(9) =13605a(10) =39174a(11) =112797a(12) =324786a(13) =935184a(14) =2692755a(15) =7753479a(16) =22325253a(17) =64283004a(18) =185095533a(19) =532961346a(20) =1534601034a(21) =4418707569a(22) =12723161361a(23) =36634883049a(24) =105485941578a(25) =303734663373a(26) =874569107070