a(n) = (5*3^(n-1)+1)/2.

Terms

a(0) =3a(1) =8a(2) =23a(3) =68a(4) =203a(5) =608a(6) =1823a(7) =5468a(8) =16403a(9) =49208a(10) =147623a(11) =442868a(12) =1328603a(13) =3985808a(14) =11957423a(15) =35872268a(16) =107616803a(17) =322850408a(18) =968551223a(19) =2905653668a(20) =8716961003a(21) =26150883008a(22) =78452649023a(23) =235357947068a(24) =706073841203