Good luck in ensuring the users retain different passwords! Else the results can be pretty disasterous - if one user mistype his/her username, webmail doesn't reject the unknown username but instead logs in to some other mailbox oin that domain. Bad enough is that one user then gets to read another's mail, but worse is that if he doesn;t notice he'sin another's mailbox (once logged in the webmail doesn't say which mailbox you're in) he can easily delete the owner's messages and send emails under the owner's name before noticing, if ever.
If anyone finds a config workaround for this, I'd be grateful to know.
I suggest the cause is the same. I.e. the mail has gone to just one mailbox, but your three logins are going in to that same mailbox, making it look to you as if three mailboxes got the same mail.