Thank you for posting your question in the Microsoft Q&A forum.
Regarding your issue, may I kindly ask a few questions to help understand your issue better:
- I saw that you posted your question for Teams under the Business category so to better support you, could you please confirm whether you're using a Home account (e.g., Gmail, Hotmail) or a Business (typically associated with a domain name like your company) account?
- Are all three existing calendars currently in Outlook, Teams, or a mix of platforms?
- Do you want the unified calendar to be in Outlook (shared mailbox calendar) or Microsoft Teams (channel calendar)?
- How many people need to see and/or edit this new combined calendar?
- Will the new calendar be for viewing only, or editing?
Hope to hear back from you soon.
The initial response may not fully address your needs, but it will help me better understand your situation. Once I get the information, I will try my best to find the next steps.
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