Another approach, that works for lists of integers.
ResourceFunction["Collatz"][3] // Select[IntegerDigits /* First@# == 1 &]
(* {10, 16, 1} *)
ResourceFunction["Collatz"][121] // Select[IntegerDigits /* First@# == 1 &]
(* {121, 182, 137, 103, 155, 175, 1186, 1780, 1336, 167, 1132, 1276,
1438, 1079, 1619, 1822, 1367, 1154, 1732, 1300, 122, 184, 106, 160,
10, 16, 1} *)