If you are confident that the standard rate and the opaque deal are the same, and both say "1 king or 2 queens", when you add the 3rd through Disney wholesale it will force the reservation to be 2 queens. I am sure there are extreme situations where people have booked, even through Disney direct and Disney has overbooked 4 person rooms where the only room type left is 1 K.
A good example would be all star resorts, where there is an option for 1K or 2Q for the base room type.
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Adding a 3rd person would force the 2 Q option.
This is where it is
absolutely imperative to be sure of what you are booking if you have more than 2. If you make the mistake of booking a room with that has the option of only 1 K or the likes and you have 3, you are SOL and the 3rd person is either sleeping on an air mattress or not going.
Let me know what dates you have and I or others will chime in.