Multiple texture mapping using 3d textures... no?

Hello I'm struggling to write my first OpenGL application and for the task I'm doing my best exploring various texture mapping techniques. I started out implementing normal mapping using shaders and now i want to add some other stuff (like height maps, specular maps and so) that i can use in my shader program. However, I read somewhere that many graphic cards only have 2 texture units so how do I manage to access 3 or more textures with my shader? My first though was to turn to 3d textures... representing one type of texture per layer... is this possible, or just a bad idea? I would really appreciate some feedback on this :)