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.

Looks like a scam email, delete.

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

Apple TV+ 3 month Offer

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

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