a(n) = (7*9^n - 1)/2.

Terms

a(0) =3a(1) =31a(2) =283a(3) =2551a(4) =22963a(5) =206671a(6) =1860043a(7) =16740391a(8) =150663523a(9) =1355971711a(10) =12203745403a(11) =109833708631a(12) =988503377683