As for adding a PS3 platform to an existing Service Account with Windows platform already, you will need to either purchase a PS3 (hard copy, or via PSN when it is available again) and enter 4-4-4 Promo code to redeem from PSN.

If you have Windows CE edition listed in Mog Stations, just get a standard PS3 will be suffice.

Things to know with PS3 version:
* 4-4-4 Promo code ties to your SEN, which when first run the PS3 game client will ask if to login using existing Square Enix Account (SQEX) or create a new Square Enix Account.
* If using existing SQEX, you will be given a list of your existing Service Account to tie your SEN/Promo code to, or a choice to add new Service Account.
* One SEN per SQEX/Service Account.
* If adding to existing SQEX/Service Account, the included subscription will be added to existing subscription time.