One of the most regular insect visitors to our pond is the hoverfly Helophilus pendulus. Today I found a pair mating. Upon closer examination it turned out to be three of them. Not something I've seen before, but as Ian put it I obviously need to get out more!
It looks like the pair underneath are mating and the male(?) above is waiting his turn. Given that he's grabbing her by the eyeballs I'm surprised that she hasn't driven him away.