As per usual tradition, all events and activities that were sponsored by The Order of The Three Races took place in a very special environment. The existence of a magical array in the basement of the building prevented the superior races from manifesting their supernatural power, not unlike Highgate Prison.
The magical array was the human beings' best protection against the superior races.
And why did human beings need protection for such an amicable banquet?
Because, as they learned in history, anything could happen when the three races were gathered in one place. Usually, human beings as the weakest race had to suffer from collateral damage.
The array proved to be useful from time to time. For example when a renowned pure-blood vampire was accused of infiltrating New Iceland, kidnapping an innocent civilian, and forcing her to spread false rumors about her birth country.