Reefwatch,
Your fears are mostly correct. Freedive foot pocket manufactures only think people’s feet are from small to large men sizes. Lots of people get left out on both ends.
With a small wife and two young daughters, this is an ongoing quest for me. Since most of us have to order over the internet to get good freediving gear, you need to become as familiar as possible with foot sizes. You said you think she is a 3. Is it 3 women’s or 3 men’s? Most freediving fins use the Eurpoean sizes. Here is a link to the best shoe size conversion chart I have found
http://www.discountshoerack.com/static/sizechart
The smallest snorkeling fins I could find are Oceanic Carbie fins size 3-4 men’s. My wife wears a size 6 women’s, which by the chart the fins should have fit her perfect, but they were still too large. Had to get her 3mm socks.
These fins are quite soft, although they do propel a light person pretty good. She is only 85 lbs and 5’ 3”. My 10-year old already has bigger feet than the wife, so she has graduated to the smallest of the freedive pockets, 38-40 with 3 mm socks. Getting thicker socks is an option.
I believe there are some fin swimming fins with smaller pockets. You could send an e-mail to Jyri at Special Fins and see if he can do anything for her. He told me once that he could have some fins custom made for my daughter, which I took to mean smaller in all dimensions.
http://www.specialfins.com
don