Hello jimfromsouthold,
To back up and make sure we're on the same page, when you say the accounts aren't showing up, are you referring to within the Calendar app only? We see you mentioned you removed and re-added the accounts, so it sounds like you see them in System Preferences > Internet Accounts, but not in Calendar. Is that right?
Also, you mentioned you have an iCloud account as well as the two third-party accounts. Are you able to sync contacts with your iCloud account?
Since the issue appears to be affecting multiple accounts, you could try booting your Mac into safe mode. This runs a basic disk check and also prevents some items from launching on startup, so it will help to narrow things down a bit. The steps will vary depending on your processor so check out this guide for the steps: How to use safe mode on your Mac - Apple Support
Once you test, click Apple menu () > Restart and let us know the results, as well as the answers to the questions above.
Take care.