Firstly, I don’t know why you are grouping by “# of plays”, this could group two different things with the same “# of plays” and apparently that’s not what you want.
Like @yamada.shoji99 said, in the RG (Repeating Group) you just need to Do a Search for music list then Sort by “# of plays” with Descending “yes” and limited to 3 items “items until #3”.
Then in the group inside the RG you just need to reference the Current Cell's music list. And for elements inside this group you need to use something like Parent's Group music list name, etc.