What can I do for an EXTREMLEY timid dog?

I have 3 dogs, The 3rd one I took from a farm in PA because she was unwanted by the woman who originally took her to breed. So I rescued her. She was 6months when I got her, she is a White German Shepherd. When I got her home it was evident she was afraid of everything! She had never been indoors even. I tried to bond with her, I tried to teach her commands. I have tried to exercises her. I have tried routine. I don't know what else to try. She is afraid of EVERYTHING! I have never had a dog like this. When she walks even she crouches down so that she isn't standing straight. Teaching her to sit was difficult because she tries to DROP to the ground. She is lovable, but I cant work with her. I have NEVER hit her. But Im wondering if she was abused? and how do I get her out of this? She jumps at every little sound, even the pop of the fireplace. She does get along GREAT with the other 2 dogs. I have had her about 6 months, and with every effort I have been able to teach her basic commands. I don't know what else to try to help her confidence? Any helpful suggestions are appreciated!