See what hits of https://www.google.com/search?q=router+measure+use+per+device are most helpful.
Three people will share one internet service. Their internet service is not unlimited. It is metered and pricey.
Albert will have just one laptop.
Bob will have a laptop and smartphone.
Charlie will have a Linux Mint desktop, a Linux Mint laptop, an Android tablet and an Android phone.
Most likely, all will be connecting via WiFi (as opposed to ethernet cable).
What do we need to see how much each person has used per month?
Ideally, each person has their own login username and password.
Ideally, each person can check their own usage for the month by going to some IP address to see their current usage. (The admin would have access to everyone’s bandwidth/network info.)
It would also be nice if there was a way to cap usage for a certain person, or when the group hits X gigabytes. (Because there may be overage fees.)