3D Textures aren't currently a core feature in OpenGL, but can be accessed as an extension. It is an EXT extension, indicating more than one vendor supports it. Even when 3D texture maps are supported, the application writer must be careful to consider the level of support present in the application. Texture map size may be limited, and 3D MIPmapping is often not supported. Available internal and external formats and types may be restricted. All of these restrictions can be queried at run time, and with care, portable code can be produced.
Consider writing your 3D texture applications so that they revert to a 2D texturing mode if 3D textures aren't supported. See the volume visualization section for an example of a 3D texture algorithm that will work, with lower quality, using 2D textures.