The current Dag size is ~2.41 GB for ETH and ~2.48 GB for ETC. I'm sure you are stil ble to mine ETH or ETC with 3GB on Linux or Windows 7 for at least another 6-9 months, just not on Windows 10. Because of the 20% GPU reserved memory space on Windows 10, the Dag file will no longer fit on 3GB cards.