Somehow, the three of them appear able to speak simultaneously and understand each other without difficulty. When they're in this mode, they speak noticably faster than normal. To an outside observer, the superposition of three very similar voices speaking at high speeds turns into something akin to white noise, with one or two distinguishable words rising above the general torrent.
Unfortunately, they seem unaware that these exchanges are nearly incomprehensible to other people, and expect those around them to have followed the conversation and be ready to act upon the consensus so reached. Baffled looks and protests of "Hold on, what??" are looked upon as signs of intransigence and obstinance.
I suspect that one or more of them may think that, because other parties are present, those parties have, in fact, contributed to the consensus decision.
I refuse to ascribe this to gender differences; I know women, including closely-related women, who have yet to demonstrate this "gestalt buzz" in my presence -- and more, I know that I have a few close friends with whom I myself can lapse into a similar rapid-fire exchange that leaves those around us baffled and blinking. However, I cannot deny that I have seen this effect depicted in numerous comedic entertainments over the years, particularly those hailing from Hanna-Barbera.