When I was in the Army, before the WMD hype, we called them "NBC" weapons: Nuclear, Biological, and Chemical.
They were grouped together in our training because they have similar battlefield characteristics, and many of the same countermeasures can be used against all three of them. We were also taught specific information about each of the three types' unique characteristics (how to recognize mild and extreme nerve poisoning, how to react to a remote nuclear blast, etc.).
So yes, "WMDs" includes all three types.
Another way to think of "WMDs" is in terms of their scope. Even the biggest conventional bomb drops in one place, and explodes in one place, and destroys only in one place. A single daisy cutter can't destroy an entire city and all the people in it. But NBC weapons cover a much larger area, and their effects are much harder control. A General can arrange to have a laser-guided (conventional) bomb penetrate a command bunker, maybe killing a handful of important enemy officers with almost no risk to civilians nearby. But he can't control the devastation caused by a nuclear bomb, or by biological or chemical agents in the same way.
While land mines don't really cause massive destruction (on the same scale as NBC weapons), the tendency of combatants to leave them behind after the war is over, so that they end up being an uncontrolled threat to the civilian population, has caused them to fall into disfavor similar to WMDs (but much less hyped right now, obviously).