a(n) = (7*11^n-1)/2.

Terms

a(0) =3a(1) =38a(2) =423a(3) =4658a(4) =51243a(5) =563678a(6) =6200463a(7) =68205098a(8) =750256083a(9) =8252816918a(10) =90780986103a(11) =998590847138