Position of n-th 3 in A031235.

Terms

a(0) =3a(1) =11a(2) =21a(3) =26a(4) =28a(5) =30a(6) =31a(7) =32a(8) =34a(9) =41a(10) =54a(11) =69a(12) =84a(13) =91a(14) =94a(15) =97a(16) =99a(17) =100a(18) =103a(19) =114a(20) =129a(21) =144a(22) =159a(23) =166a(24) =169a(25) =172a(26) =174a(27) =175a(28) =178a(29) =189