mac calendar not syncing with iphone & ipad via itunes

If you use iCloud, I don't think calendar (plus contact and reminder) data goes through an iTunes sync to iOS devices, such as iPhone and iPad. It goes through iCloud. You need to be signed in using the same Apple ID on your Mac and iOS devices. Updates are seem in iCloud by all associated devices and computers (for an Apple ID). iTunes is not involved. This document has details

Get help using iCloud Contacts, Calendars, or Reminders - Apple Support

I realize what you said and desire, but you just recently updated your Mac's OS and your iPhone's OS with major releases. So you should confirm settings are as you expect. On the Mac, open System Preferences iCloud pane. If you do not want to use iCloud, you should not be signed in there with your Apple ID. Or Calendars (and maybe also Contacts, Reminders, and Notes) should not be checked (if you are signed in with your Apple ID for iCloud in general).

On your iPhone, you are signed in with your Apple ID. As a test, make a change to your calendar on your iPhone. Then, on the Mac, open Safari and go to iCloud.com. Sign in there, with your Apple ID. Click Calendar. Is that change you made on the iPhone reflected in your iCloud calendar? If so, your iPhone is using your iCloud calendar, even if your Mac is not.

If you are using iCloud for your calendar (and those other items), you do not sync that data using iTunes anymore. Your Mac updates your iCloud calendar, and your iPhone updates your iCloud calendar. Your Mac and iPhone should both access the same iCloud calendar directly. iTunes is not in the picture. So the actual question is, why didn't the iPhone calendar update your iCloud calendar?

Are you using the same Apple ID on the Mac and iPhone, and you used that same Apple ID to sign in at iCloud.com? Your iPhone has an active Internet connection?

Second test... If you update your calendar on the Mac (using Calendar app), then look at your iCloud calendar on iCloud.com (using Safari), is THAT change reflected?

I just added two test events on my calendar, one each on my Mac and on my iOS device. Both test events now appear on my iCloud calendar (as viewed at iCloud.com), AND I can see both events on my Mac and iOS device, using their calendar apps.

Sorry, I misread what you wrote...

So, what happens if you turn ON iCloud for calendar on your Mac, but not your iPhone? Do you still get the "3 sets of duplicates" issue?

In that case, what happens if you turn ON iCloud for calendar on your iPhone, but not on your Mac?

Maybe the duplicate issue happened, because it "merged" your calendars on your Mac and iPhone, which had the same entries. If you only enable one device, the one with the up-to-date entries, does that duplicate issue still occur.

Does your iCloud calendar already have entries? Or is it currently "blank"?

Ultimately, you may need to enable iCloud calendar, so that you Mac and iPhone can use the current method of immediately seeing the same calendar data on all associated computers and devices.