Hello, this is Dr. Linda. I hope I can help with your question today.
I am very sorry about your concerns about Annabelle. Most likely she has a whopping case of shipping fever or Bovine Respiratory Disease Complex. Hopefully it has not progressed to pneumonia after 3 weeks. Without knowing what your veterinarian sent home with you 3 weeks ago, you need to get in contact with him again to get Annabelle on some of the big guns of antibiotics for the respiratory disease that she has, such as Draxxin, Micotil, Nuflor Gold or Baytril. Anything from the feed store will not cut it at this point, so do not even think of giving her penicillin, or LA 200. It will be a waste of money.
The best advice I can give you would be to have her examined by your veterinarian. 3 weeks is a long time to be sick. I know you said it is difficult to get her to the vet, can he make a farm call? If Annabelle's respiratory disease progresses too far, she may end up with scarred lungs & she will always be an unthrifty animal. Also, at this point, the B12 shot should be reserved until after you know more about what is going on.
Unfortunately, any over the counter treatments will just not be sufficient. Her respiratory disease is either very deep seated or possibly even resistant by this time. I hope you can have her examined soon.
I am sorry that I can not advise any other remedy except for contacting your veterinarian, but I do hope this is information that helps. Please let me know if you have any further questions or concerns.