Is there a NOP texture function available?

In order to make available three different textures to the multitexture pipeline, I must enable three associated texture units. However, I only need a single texture combiner function for my rendering. Is there a NOP texture function available that I can setup for my remaining two texture units to maximize the performance for the render pass? If there isn’t one explicitly available, what is the highest performance pass-through texture function I can use for the remaining two passes?

Thanks for any help.