a(n) = (7*5^n - 1)/2.

Terms

a(0) =3a(1) =17a(2) =87a(3) =437a(4) =2187a(5) =10937a(6) =54687a(7) =273437a(8) =1367187a(9) =6835937a(10) =34179687a(11) =170898437a(12) =854492187a(13) =4272460937a(14) =21362304687a(15) =106811523437a(16) =534057617187