Position of n-th 3 in A031253.

Terms

a(0) =2a(1) =12a(2) =13a(3) =15a(4) =17a(5) =22a(6) =37a(7) =48a(8) =51a(9) =52a(10) =67a(11) =82a(12) =84a(13) =87a(14) =90a(15) =97a(16) =98a(17) =101a(18) =104a(19) =107a(20) =110a(21) =112a(22) =113a(23) =116a(24) =119a(25) =122a(26) =123a(27) =125a(28) =126a(29) =127