Most likely the other two accounts have been disabled.
You can only fix it if you've got admin rights, and seeing as yours is the only account you can log into, I'd say you'd need to go to the control panel and set them up again in user accounts. If it doesn't work - eg, you've been to the control panel but can't get the accounts set up again because it gives you crap about not having admin rights to do so, then you'll need to probably enable the administration account that hardly anyone ever sees when they set up their own account for the first time. If that's the case *goes to look at fixes folder (I got a few I can tell you)* ah, here we go....you'll need to go to the start menu, tap in 'cmd' minus the quotes, wait for it to pop up in the start menu, right click on it and run it as admin, then type the following.....no quotes..."net user administrator /active:yes"...it's important to get the spaces right! Hit enter and it should tell you the admin account has been activated in not so many words.
Reset your computer, choose the admin account - if it's got a password you'll need to know it obviously, then try to set up the accounts through that in the control panel.
So long as that works you should be okay. You'll see it in there it's pretty self-explanatory when you get into the control panel. Now you'll probably want to disable the visibility of the admin account on the log on screen....reset the computer, go to your own account, same as before - type in cmd etc, only when you get the command prompt window, instead of typing "/active:yes" you type "/active:no", don't forget the space between 'administrator' and "/" otherwise it won't work.
Reset the computer once more and any accounts you've set up again should show up. If they don't, then the problem is probably much more serious than indicated.
Obviously this whole thing only works for Windows....as you didn't specify which OS you were using.