How to generate list of externally shared sites, externally shared lists and external users?
Perform the following steps to generate the Reports:
Select Sharepoint Online Reports from the available vertical tabs. Then from left pane of application, select External Sharing.
On the selection of "List of External Shared Sites" from the list of available reports, the resultant data is displayed as shown below:
 

     
On the selection of "List of External Shared Lists" from the list of available reports, the resultant data is displayed as shown below:
 

     
On the selection of "List of External Users" from the list of available reports, the resultant data is displayed as shown below: