I only learned quite recently what a 'three-bird flight' is. When you're hanging around near lakes and ponds at this time of year, you may notice Mallards flying about in threes - two males and one female. This is a three-bird flight - an established pair of Mallards plus an unmated drake trying to muscle in on the female. In the flight, the pair are trying to shake off the interloper or at least not let him get too near the female, who will often be loudly quacking in alarm.
On Wednesday I spent a few hours in a certain London park between a work meeting and a long-overdue gathering with the Gentlemen of Science, and saw quite a few of these three-bird flights, not all of them involving Mallards either.