I am SO happy for the 3 little ones, and so glad that you are caring for them. Since they can't say it, I will: THANK YOU!

Since the mom is so wary of you - of humans, not just you - getting her used to things "may" help the process. If she is a true feral, then being in an enclosed area will be scary initially. Is there an open porch or area with a roof or covering where you could put a litter box? Maybe with something scented from her kits in it? Do you have a cat carrier you can leave out there with a food dish in it, so she gets used to walking inside?

Maybe a local pet store, hearing this story, would volunteer a trap? Or even a large dog kennel? Keep the kittens in there for a bit with an old towel, to set their scent and then put it outside for her to investigate?

I am grasping at straws here. I got frustrated reading that none of the rescue groups has been able to get in touch with you. They would have some suggestions. Continued GOOD LUCK!!