Looking at the big picture, the new GTX 1060 3GB materially differs from the existing 6GB GTX 1060 in two different metrics. First is the memory; the card ships with half as much memory.
Whereas the original GTX 1060 6GB shipped with a fully enabled GP106 GPU, the GPU used in the GTX 1060 3GB ships with 1 of the 10 SMs disabled. This leaves 9 SMs enabled, leading to a CUDA core count of 1152, and 72 texture units. Other than this sole disabled SM, the GPU is otherwise untouched, and the full ROP/L2 backend and its associated memory controllers are fully enabled.
Clockspeeds are also unchanged. On the GPU this means we’re still looking at 1506MHz base and 1709MHz boost. Meanwhile on the memory it’s still 8Gbps GDDR5 on a 192-bit memory bus, only now there’s only half as much total memory.
On the TDP side, TDP hasn’t been adjusted even with the loss of 1 SM. This means TDP remains at 120W.
MSI, Gigabyte, EVGA, and others are already listing cards on online stores.