`w U1` - login informations for example for user U1 you can see in output something like this
User tty login@ idle JCPU PCPU what
also you can make a bash script and put in crontab for example to be run a few times per one day, with varibles w U1, w U2, w U3 and make some otput into file also
`/var/run/utmp` - List of current login sessions.
`/var/log/wtmp` - List of previous login sessions
`/var/log/btmp` - List all the bad login attempt