a(n) = 6*a(n-1) - 9*a(n-2) + a(n-3), with a(0)=3, a(1)=6 and a(2)=18.

Terms

a(0) =3a(1) =6a(2) =18a(3) =57a(4) =186a(5) =621a(6) =2109a(7) =7251a(8) =25146a(9) =87726a(10) =307293a(11) =1079370a(12) =3798309a(13) =13382817a(14) =47191491a(15) =166501902a(16) =587670810a(17) =2074699233a(18) =7325660010a(19) =25869337773a(20) =91359785781a(21) =322660334739