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Three or even just two nesting boxes should be fine for six hens, although unless you leave a dummy egg (or golf ball) in each nest, your hens will probably all want to lay in the same nest.
I think this is adaptive behavior. A hen that still has the instinct for broodiness will begin to incubate a clutch of eggs when there are "enough" eggs accumulated in that nest. What's "enough" depends on the particular breed of chicken. If one of the big girls that sits on clutches of 18 eggs waited until she herself lay 18 eggs to make up that clutch, the first egg would be at least 18 days old, and not very likely to hatch. However, if her flock mates pitch in, a clutch of 18 eggs might only take 3 or 4 days if the flock has six hens in it. The eggs will be fresher when she starts to set them. That's my guess about why hens lay in communal nests, and why they look for a nest that already has an egg in it when they look for places to lay.