Might it be possible that the 3 sites you are missing don't use extracts?
Good Interpretation but we are using extracts in those sites also. I can view their status individually in sites but as a whole from the server end its not possible.
I took my example from Tableau Server 10.2, but the behavior on server 9.2 is the same:
You can not see all your sites in the dropdown of 'Sites' because of the combination of the filters.
Could it be that you are maybe only showing the extract refreshes of the last time, while some sites only refresh their extracts weekly?
I would play with the combination of the filters in the 'Background tasks for extracts' view.
You're correct . I changed the timeline and did a check. I'm able to view those sites now. Thanks for the response and it was really helpful