Position of n-th 3 in A031067.

Terms

a(0) =3a(1) =16a(2) =21a(3) =33a(4) =34a(5) =35a(6) =41a(7) =55a(8) =69a(9) =80a(10) =98a(11) =106a(12) =108a(13) =117a(14) =118a(15) =123a(16) =130a(17) =132a(18) =141a(19) =145a(20) =149a(21) =161a(22) =178a(23) =193a(24) =195a(25) =216a(26) =229a(27) =234a(28) =238a(29) =242