These are the hoverfly Eristalis nemorum. The one on the flower is a female, you can tell because her eyes do not meet on top of the head. The two hovering above her are males - you can see their genital capsules at the end of their abdomens. We are not sure whether this is courtship or mate guarding - i.e. one of the males has already mated with her and is preventing another male nipping in before she has had a chance to lay eggs. Since it is quite common to see more than one male in these situations, courtship seems more likley - but mating following this has not been observed - at least not so far reported to the Hoverfly Recording Scheme!