None of the three are a great play.
Randle could be a high-risk, high-reward play, if Murray gets hurt, but passing downs are going to belong to Dunbar.
Williams is part of the Arizona backfield...yuck...but might be a better candidate to get more carries than Randle. Still might split with Taylor.
Thompson is also a backup, but Denver is very run-heavy these days.
Safe play might be Williams, but Randle / Thompson could be sneaky plays. Randle would have the bigger upside if Murray had complications. I'd go Williams to be safe, but Randle if you need to swing for the fences.