a(n) = 3*a(n-2) - a(n-3), with a(0)=3, a(1)=0, and a(2)=6.

Terms

a(0) =3a(1) =0a(2) =6a(3) =-3a(4) =18a(5) =-15a(6) =57a(7) =-63a(8) =186a(9) =-246a(10) =621a(11) =-924a(12) =2109a(13) =-3393a(14) =7251a(15) =-12288a(16) =25146a(17) =-44115a(18) =87726a(19) =-157491a(20) =307293a(21) =-560199a(22) =1079370a(23) =-1987890a(24) =3798309a(25) =-7043040a(26) =13382817a(27) =-24927429a(28) =47191491a(29) =-88165104