I had a friend who had a terrible experience with a feral cat who gave birth in captivity. However, it seems you have 3 weeks to try to gain the trust of this Momcat, my friend had none...and I think you can make FABULOUS headway in 3 weeks, especially if you already have her in an enclosed area.
Be The Bringer Of Good Things. Tasty food, especially. Try to get her comfortable eating near you, possibly put some cooked chicken or canned food on your fingertips to offer towards her (ONLY if she isn't yowling and prepared to attack or swipe you with her claws
). The closer you can get her to come to you, the better. If she will allow you to lightly touch her by halfway through the 2nd week or early in the 3rd week, I think it should go fairly well.
#1, need to stay safe.
#2, you really need to try to get as much progress made before kittening as you can.
#3, try to keep every experience as positive as you can so you keep building on the Good Things.
If you can get to the point of lightly touching her before birth, I would back off for the first few weeks after she has her kittens, to allow for her mother-instinct to relax and learn she and her babies will be SAFE with you, and then you can begin with the socialization again. By the time the kittens are about 3 to 4wks old, they should be tumbling about and beginning to explore and if the Mamma is okay with you being in there, the babies won't learn from her to stay away from you.
I feel kittens don't learn that distrustful behavior until they reach about 6wks of age, and if you are a positive experience for them and bringing them canned food at weaning time (between 4-6wks) they will learn to love you no matter what Mamma says.
I'll be hoping she warms up to you quickly...