OP your boss may be a bit behind on things. there is now a published page on TW about in-office expectations (popped up recently) making it super-clear that you can be fired. They aren't going to send around some big announcement. it's been communicated thru layers of mgt.
In 2022 it was optional for managers to enforce/create consequences.
In 2023 it's not optional. if you are a manager and your team isn't coming in - it'll be taken out on your (manager's) performance.
The page in TW also says that it's not any one specific week, but adherence over time. There is latitude, just as there was with 5 days in-office, for people to have weeks where they don't meet the three days due to circumstances that are agreed to between the employee and the manager. (e.g. sick WFH, catastrophic weather, etc)
The reporting shows rolling four weeks. (I'm a manager and i'm getting it regularly). It is day by day and to the individual name, as well as summarized stats.
Go read teamworks everyone. you have been considered officially notified with that page!