Position of n-th 3 in A031111.

Terms

a(0) =3a(1) =9a(2) =10a(3) =30a(4) =37a(5) =61a(6) =72a(7) =82a(8) =85a(9) =89a(10) =90a(11) =93a(12) =96a(13) =97a(14) =104a(15) =120a(16) =138a(17) =140a(18) =144a(19) =150a(20) =178a(21) =179a(22) =182a(23) =186a(24) =200a(25) =210a(26) =219a(27) =226a(28) =238a(29) =241