This is a very odd case, usually you get only one if the first user happens
to get exclusive access to the database, or many if shared. The theoretical
limitation for Access is 255.
What is the architecture? Where is the back-end, where are the front-ends?
Are all the FEs correctly linked to the BE if you logon with them
individually?
Have you verified that it is always precisely two? Is it two in particular
or will any two users do it? Does it make any difference which machine(s)
they use?
It would help if you could say in detail what happens when the third tries
to get access. Is there an error message? What exactly happens?
David