3 month free trials disappeared

Here are the terms: New and qualified returning subscribers only. $9.99/month after free trial. Only one offer per Apple Account and only one offer per family if you’re part of a Family Sharing group, regardless of the number of devices you or your family purchases. This offer is not available if you or your Family have previously accepted an Apple TV+ 3 months free or 1 year free offer. Offer good for 3 months after eligible device activation. Plan automatically renews until cancelled. Restrictions and other terms apply.

I suspect you waited too long to activate the trial, and the offer expired.

You can ask Apple about this, as they’re the arbiter: Official Apple Support

To claim your offers, open the corresponding app on your qualifying Apple device (iPhone in your case), sign in if not automatically signed in, and find your offer acceptance button there. The previously seen offer notifications were just shortcuts to the page in those apps.

Current Apple offers - Apple

Apple has been a bit loose with the claim period, sometimes citing 3 months, sometimes citing 90 days. And obviously there’s a few days difference there. So better not wait until the last possible moment.