a(n) = 4*a(n-1) - a(n-2) for n>1, a(0)=3, a(1)=9.

Terms

a(0) =3a(1) =9a(2) =33a(3) =123a(4) =459a(5) =1713a(6) =6393a(7) =23859a(8) =89043a(9) =332313a(10) =1240209a(11) =4628523a(12) =17273883a(13) =64467009a(14) =240594153a(15) =897909603a(16) =3351044259a(17) =12506267433a(18) =46674025473a(19) =174189834459a(20) =650085312363