Most often you are able to select seat after having entered your name and e-ticket ID number (or ticket number).
There is a link to "Select Seat" on the page that shows flight details etc before you click on the Check in button.
Otherwise you are automatically designated a seat if there are pre-assigned seats on this flight.

The BN3 is simly the sequence number - i.e. you are number 3 to have checked in on this particular flight.
The seat is usually also shown on your printout together with the flight details.

It is always possible to enter the check in online again and then you will be informed that you are fully checked in and that you can either cancel the check in or select another seat. The seat map will then be pulled up.