Forum Discussion
Sync multiple calendars in one
Maybe MS Flow can help. Check https://flow.microsoft.com/en-us/templates/?category=eventsAndCalendar&sort=properties%2Fstatistics%2Finstantiations.
It is hard to sync calendars in both ways, and in outlook there is no trigger 'When event is deleted', but all new meetings you can send automatically to one calendar.
If you want to sync data from multiple mailboxes (eg calendars) into one, you could give Exchange Sync by Connecting Software a go:
http://www.connecting-software.com/cb-exchange-server-sync/
If you don't want to spend any money on this, you need to resort to means offered by the mail platforms used. GMail is the odd one out, I'm afraid there's no 'automated' process offered out of the box except for following strict manual procedures. If it's a one-way sync, you could develop something yourself, eg clear the overview calendar, and import/merge the source calendars using imap (as GMail doesn't support EWS).
Since the tools mentioned solve a business problem for you, in addition to being supported tools, there is a business case making the price worth compared to manual labor.