I got an email offering a 3 month Apple TV subscription with our new iMac. but when I click on accept 3 month free, I get a webpage that says 7 days free.

Easy, because you are not redeeming the correct offer. I assuming you have never subscribed to Apple TV + (please note this is NOT for the Apple TV app, it is only for Apple TV +) To learn how to redeem the offer please click Apple TV+ 3 month Offer. The first thing to check to see if you are eligible for the offer or not.

You have to redeem the offer, however the offer is only valid if you qualify for it.

Apple TV+ 3 month Offer

Looks like a scam email, delete.

I was offered 3 months Apple TV after I purchased a new iMac.

It offers 7 days. Why is this not available now?