I personally think a 3/8" rolled hem double stitched would be much more asthetically pleasing than a 3mm for this application. I do think both would work though.
I thought I should add that 3mm rolled hems are what you commonly see on the hems of satin layers on dresses. Two layers in a 3mm foot will be difficult to make look nice, but it can be done.
--If a cow laughs hard, does milk come out its nose?