There are points to be made for each side of this question.
On the one hand, all of these kinds of weapons are clearly different from what we call "conventional" weapons. They all have a much greater potential to spread death across a wider area than could possibly be affected by the explosion of even the largest cluster munitions or field of land mines. In this sense, they all belong together in a distinct category.
On the other hand, these weapons are not all that similar to one another. Biological weapons are much harder to create, but they could cause devastation much greater than the others if they are made in the form of an infectious disease. Such a biological weapon could conceivably affect whole countries, which is something that chemical or nuclear weapons could not do.
On balance, it is best to classify these together as they are all very different from conventional weapons. It might possibly be useful to further break down the classification of WMD, just as we break conventional weapons down into things like bombs, artillery, and guns. However, this does not detract from the fact that all of these types of weapons are qualitatively different from conventional weapons.