Mullerian Mimicry: A Study in Bad Taste
They could be triplets, but each of these three butterflies belongs to a different species.
If they are different species, then why do they look so similar?
This is an example of Mullerian mimicry. This type of mimicry occurs among two or more harmful, toxic, or in this case bad-tasting species that are eaten by the same kind of predator.
Incidentally, all of these butterflies taste terrible because of the plants they eat: Monarchs and queens eat bitter milkweed plants; viceroys eat the bitter leaves of certain trees.
The point is that a bird that has a bad experience tasting any one of these butterflies like the viceroy is more likely to avoid the other two species, monarch and queen. Therefore this arrangement reinforces the protective colors of all three species.