First you need to take a deep breath. Luxating patella is not the end of the world. In my mind the spinal issue is the priority right now. Keep her on crate rest and yes, medicate her for pain but only with Tramadol. Don't use anything like Rimadyl or Metacam with her being on prednisone because she could bleed.
There are some ways to get funding for vetting. The one that I hear the best results with is
IMOM Home
There is a lengthy list of other organizations here:
Having Trouble Affording Veterinary Care? : The Humane Society of the United States
Grade 3 LP does not always need surgery. She is older and the surgeon I use does not repair them unless they are really an issue and causing pain. Temporary pain like you are describing sometimes will ease up. If she has torn a ligament that could be a different issue, but for right now, I personally would let her rest .. and look into funding help. Perhaps you will qualify!
Keep us posted! I will keep her and you in my thoughts and prayers.