Position of n-th 3 in A031087.

Terms

a(0) =3a(1) =15a(2) =33a(3) =46a(4) =48a(5) =50a(6) =51a(7) =52a(8) =54a(9) =56a(10) =58a(11) =60a(12) =62a(13) =69a(14) =87a(15) =105a(16) =123a(17) =141a(18) =162a(19) =189a(20) =216a(21) =235a(22) =238a(23) =241a(24) =243a(25) =244a(26) =247a(27) =250a(28) =253a(29) =256