If they're simply singing the song unadorned then its not an arrangement. I don't think simply selecting verses (or choosing the order of verses) taken from collected sources is sufficient to be an "arrangement" for copyright purposes. It's only an arrangement if instrumental or vocal parts are added.

If A simply sang the song without instruments or other vocal parts, and then B arranged it for instruments and/or voices, and C then copies B's arrangement, it should be Trad arr B. A doesn't get a mention.

IMHO