Previously we were told by premier staff that having a third person join us in our room would be no problem as it's done all the time. Finally our lady friend has decided to join us and I am trying to get my booking done but being told that they cannot do this and I need to wait until I get there to speak to the premier folks. A little concerned obliviously that the 3 of us show up and they still won't allow it.
I should also add that we upgraded to the Seduction suite when we were told no problem but unfortunately didn't have them write it into the contract
They allowed us to have 3 in a tower room back in February but that was an upgrade from Garden View so YMMV. When we bought our membership we had them write addendums that specified 3 people instead of a couple for everything so we'd be covered in exactly the situation you're in. Unfortunately if it's not written in the contract it's really at their discretion, hell even with a written contract we have to argue with them sometimes.
They have allowed 3 to a room (maximum) for as long as I recall and the 3rd person just pays the AI fee. I would have a real problem not knowing what that fee is going to be until I got there because you would have zero leverage and they could charge you whatever they wanted and it does fluctuate and is negotiable. I wouldn't consider doing this until they can confirm the rate for the third.
2 to a room with a single bed. 3 to a room with two beds. That's the standard for how they will book rooms, assign rooms, and charge for rooms (which names are associated with which room). Hence no three people to a room or two people wanting twin beds in the towers. But they will allow you to have three in a room as long as on paper your 3rd is booked to another room.