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.