It’s not “sessions per day” (or per week, or per month) it’s “concurrent session”.
WordPress has the ability to keep session even if the user doesn’t interact with the session for many times.
If you go in the page PerfOps Tools > Sessions, you will see many sessions which are “unused”.
If you want these unused sessions “vanish”, you can set an idle time…
But sessions ans sessions limitations are definitely “concurrent sessions”.
So, “3 sessions per user” means a user can have up to 3 simultaneous sessions (regardless it’s ip, device, etc…).
Is it more clear?
(don’t hesitate to ask if it’s not, English is not my mother tongue ?? )