Typically when we see different variants of the same card the only thing that is different is the amount of memory (VRAM) on the card, but NVIDIA is changing things up with their upcoming GeForce GTX 1060 3GB model. If you remember NVIDIA launched the 6GB GTX 1060 not that long ago. With the 3GB model the card will not only have less memory, but also fewer CUDA cores.
The 3GB SKU is positioned to compete directly with the Radeon RX 480 4GB at a price point of $200. The 6GB version of the GTX 1060 has 1280 CUDA cores spread across 10 streaming multiprocessors (SM), whereas the 3GB variant will have only 1152 CUDA cores spread across 9 SM. This could also lower the TMU count from 80 to 72.
The clock speeds to appear to be unchanged so you will have 1506 MHz core with the GPU boost up to 1709 MHz, and 8 GHz (GDDR5-effective) memory, giving you 192 GB/s of memory bandwidth.