Position of n-th 3 in A031035.

Terms

a(0) =3a(1) =15a(2) =31a(3) =40a(4) =42a(5) =44a(6) =46a(7) =47a(8) =48a(9) =50a(10) =52a(11) =54a(12) =63a(13) =79a(14) =95a(15) =111a(16) =131a(17) =155a(18) =179a(19) =193a(20) =196a(21) =199a(22) =202a(23) =203a(24) =205a(25) =208a(26) =211a(27) =214a(28) =227a(29) =251