Position of n-th 3 in A031312.

Terms

a(0) =2a(1) =9a(2) =19a(3) =26a(4) =28a(5) =29a(6) =30a(7) =32a(8) =34a(9) =39a(10) =49a(11) =59a(12) =69a(13) =79a(14) =89a(15) =101a(16) =116a(17) =131a(18) =142a(19) =145a(20) =146a(21) =148a(22) =151a(23) =154a(24) =161a(25) =176a(26) =191a(27) =206a(28) =221a(29) =236