Position of n-th 3 in A031018.

Terms

a(0) =2a(1) =15a(2) =18a(3) =20a(4) =22a(5) =24a(6) =29a(7) =43a(8) =63a(9) =80a(10) =83a(11) =84a(12) =86a(13) =105a(14) =126a(15) =147a(16) =152a(17) =155a(18) =158a(19) =161a(20) =168a(21) =189a(22) =193a(23) =196a(24) =199a(25) =202a(26) =205a(27) =208a(28) =210a(29) =211