(5*7^n+1)/2.

Terms

a(0) =3a(1) =18a(2) =123a(3) =858a(4) =6003a(5) =42018a(6) =294123a(7) =2058858a(8) =14412003a(9) =100884018a(10) =706188123a(11) =4943316858a(12) =34603218003a(13) =242222526018