Can't say anything for sure, but considering beta phase 3 states that all holders of XIV service accounts are eligible to play, you may not even get an invite sent to you, and instead it may just be automatically added to your account. Can't confirm it, like I said, but just a guess since it would be a lot of emails for SE to send out just to send codes to everyone with an active or inactive XIV service account attached to their SE Account when they could simply flag all XIV service accounts with beta access.