Position of n-th 3 in A031219.

Terms

a(0) =3a(1) =12a(2) =22a(3) =25a(4) =27a(5) =29a(6) =31a(7) =32a(8) =33a(9) =42a(10) =56a(11) =71a(12) =86a(13) =91a(14) =94a(15) =97a(16) =100a(17) =101a(18) =103a(19) =116a(20) =131a(21) =146a(22) =161a(23) =166a(24) =169a(25) =172a(26) =175a(27) =176a(28) =178a(29) =191