Position of n-th 3 in A031298.

Terms

a(0) =3a(1) =16a(2) =36a(3) =51a(4) =53a(5) =55a(6) =56a(7) =57a(8) =59a(9) =61a(10) =63a(11) =65a(12) =67a(13) =69a(14) =76a(15) =96a(16) =116a(17) =136a(18) =156a(19) =176a(20) =199a(21) =229a(22) =259a(23) =281a(24) =284a(25) =287a(26) =289a(27) =290a(28) =293a(29) =296