NVIDIA will offer a 3GB version of the GTX 1060, and there's more to the story than the obvious fact that is has half the frame buffer of the 6GB version available now. It appears that this is an entirely different product, with 128 fewer CUDA cores (1152) than the 6GB version's 1280.
Image credit: VideoCardz.com
Boost clocks are the same at 1.7 GHz, and the 3GB version will still operate with a 120W TDP and require a 6-pin power connector. So why not simply name this product differently? It's always possible that this will be an OEM version of the GTX 1060, but in any case expect slightly lower performance than the existing version even if you don't run at high enough resolutions to require the larger 6GB frame buffer.