a(n) = (5*11^n+1)/2.

Terms

a(0) =3a(1) =28a(2) =303a(3) =3328a(4) =36603a(5) =402628a(6) =4428903a(7) =48717928a(8) =535897203a(9) =5894869228a(10) =64843561503a(11) =713279176528