Windows only: To play three characters simultaneously
- A Square Enix account
- A copy of the Windows version
- Three registration codes obtained from the product or the "FINAL FANTASY XIV Service Center"
*You can register up to eight registration codes per Square Enix account.
*The "FINAL FANTASY XIV Service Center" website will be available after the game is launched. Please note that the website can only be accessed by those who purchased FINAL FANTASY XIV and registered a service account.
This one already saying you can multibox with 1 SQEX ID, reason why they do not specify this quote :

*Please create separate service accounts for the Windows version and the PlayStation 3 version of FINAL FANTASY XIV. You will not be able to play at the same time if you register the codes under the same service account.
Was because there isn't a need to do so for PC version, SQEX will detect the code and place it as a new Account Service.