With the cooler weather here it is a good idea to put in the entrance reducers. The cooler weather will have the bees clustering in the hive. In a three deep hive the bees will be mostly in the middle box. So not many bees in the bottom box.
Mice are looking for a place to stay for the winter. Entrance reducers will keep them out of the hive.
Entrance reducers should be placed at the larger opening.