I wish Neil was still around tonight; he could answer this much better than I.
As he pointed out in the post above, the third and fourth passenger can sometimes pay the same as the first and second. But they don't start with a set price for the cabin and then just divide by the number of people. For example, the single supplement (a single traveling alone in a cabin for two) is usually 150% in the less expensive cabins; 200% in the better ones. Very often the third and fourth person will pay 50% of the fare for each of the first two.
In most brochures the fare listed is "per person, double occupancy".
Are you sure the prices you were quoted were for all three people, or for two people in a cabin that can hold three?
Maybe the agency is trying to get the most it can out of a cabin that will hold three.
In any event, that is not the way prices are usually decided--setting a price for a cabin and dividing by the number of people in the cabin. Normally they set a price per person, then see how they can maximize the revenue from each cabin.