Get 6GB ones clearly. 3GB is pushing it, it is plenty enough for now, but there is at least one game already capable of using more than that in 2K resolution.
3GB is pretty standard now, really. And as you know, what is standard now will become weak later, so it only makes sense to have your ass covered in case VRAM heavy titles start popping out. Watch Dogs is the first of many, really.
I am pretty sure you want your SLI setup to last you 3 years or so, so prepare accordingly.