It seems 3GB cards won't be able to mine ETH/ETC in Windows10 soon because this OS reserves about 20% of GPU memory size and there is no way to fix it (at least easily).
In next update I will set zero devfee for these cards (same as I did for 2GB) so you can mine ETH forks.
If you still want to mine ETH/ETC on 3GB cards you should use Windows7 or Linux. Windows
Since Windows 7 is limited to 4 cards, I think Windows Server 2016 is another option for using Windows if you have more than 4 cards. There is also a 180-day evaluation of Windows Server 2016 for free.
https://www.microsoft.com/en-us/evalcenter/evaluate-windows-server-2016