Position of n-th 3 in A031027.

Terms

a(0) =3a(1) =9a(2) =31a(3) =43a(4) =55a(5) =57a(6) =65a(7) =70a(8) =80a(9) =88a(10) =97a(11) =103a(12) =111a(13) =116a(14) =117a(15) =123a(16) =131a(17) =147a(18) =150a(19) =151a(20) =156a(21) =170a(22) =176a(23) =182a(24) =190a(25) =195a(26) =200a(27) =205a(28) =217a(29) =221