Leave her kittens alone. Don't touch them. We just had the same thing happen. I found out last night my nephew was touching her kittens and so she moved 2 of them, but left the 3rd behind. The momma was a feral (wild) cat and so he instincts told her that her kittens we’re in danger. If she feels threatened at all by you, she'll move them. Best thing you can do is leave them totally alone until they are about 4 weeks old, then when momma leaves to go hunting, take the kittens. They will have to be bottle fed, so I would contact a rescue group before you take the kittens so that they can have fosters ready for them.
Of course, if you have a good humane society, they'll send someone out with a trap and they'll place the kittens inside. When momma goes in, they'll all be trapped and can then be placed for adoption. Good luck!

Here's a very helpful site that YLL gave me:
http://www.kittenrescue.org/handbook.htm