Hello, I'm Dr Ralston, thanks for your question.
It sounds like this cat has always been skittish or nervous. You have said she spent most of her time under the bed of the previous owners.
Some cats are just nervous cats from the beginning. It can be like personality traits in a person. Shyness for example. Take in mind certain large cats like panthers, pumas, bobcats, lynx, etc. These are all cats that like solitude. They prefer to hunt alone, and be by themselves. They can sneak around better and be more succesful that way. We would kind of describe them as shy wouldn't we? And why should it be any different for your housecat? Some cats are gregarious and want to be in public and around other cats, and some cats would rather not be around any other cats.
There can be many other reasons why they are like that. In some cases I believe it is psychological. Perhaps they had a bad experience with other cats when they were younger. Maybe the cat was a street cat at some point for example and had to fight for food or shelter with other cats. This would create a normal sense of distrust and anxiety around other cats.
But, fear towards humans, and environment, and situations can be due partially to socialization. It may have been noisy in the previous environment or there was an odor that bothered the cat. Cat's are very complex. At any rate,cat psychology can only take us so far. There is much we don't know about the way cats think. Even if we could ask them, it might not make sense to us.
But we also have to rule out physical causes of fear or anxiety. It sounds like your cat has been checked by the Veterinarian. That is a good start. I always tell people to have complete blood tests and urine tests completed to rule out metabolic causes, and urine to rule out things like urinary stones and crystals, or infections that can be present for years. These can cause much pain and anxiety for the pet over time. They strike out towards those that are near because they do not understand where the pain is coming from, and to them, that pain is coming from you, or another cat for example. It might not make sense to us, but I believe that is "Cat Logic". It seems that when the source of pain or discomfort can be found or eliminated, the behavior of the cat often improves. So, because of this, I believe that is often the truth.
I have seen similar problems when the pain is caused by a bad tooth. Many Vets I feel do not examine the teeth closely enough in cats, especially if the cat is aggressive, or acts as if it might bite. Can you blame them???? But, I have seen many cases of cats with resorptive lesions (FORL's) on their teeth. These are when the gum of the animal basically starts to eat the tooth due to trauma, or autoimmune problems, or infection. These are very painful. IN some cases, even if the Vet looks in the mouth, they will not see these sources of pain, because they might even be below the gum line, and it takes an x-ray to sort it out.
I have seen cats that have pain from their skin - an abnormal neurologic type of pain that causes them to sometimes twitch, or strike out, especially when being petted. Anti seizure medications have been helpful in these cases to sedate the nerve ending that are causing these sensations.
These are all just examples to illustrate the point that many possible causes for the cat's behavior can be explored.
In some cats, you will not be able to help this problem. IT might be too late after three years.
You are doing the right thing by giving her treats to help her come out of hiding, and rewarding her when she does. Don't force anything, you will just make it worse.
So, be sure to have her checked to rule out any and ALL physical problems. Doesn't hurt to get a second opinion from another Vet either. You never know when something might have been missed.
A product you might wish to use which helps to alleviate stress and provide calm in cats is called Comfort Zone by Feliway. (CLICK HERE)
We have also used valium and buspirone in some cats to relieve anxiety. Many people are not comfortable using medications like these in cats. They don't want to "dope" their animals. These medications are very safe, valium it seems is quite enjoyable in cats, and consider the pain and stress cause from the anxiety and nervousness. The fear these cats have every day? The medications are a blessing to these poor pets. So, consider that and speak with your Vet about it.
One last thing I would like to leave you with is from my files. It is about working with cats and introducing them to new people. Again, it's from my files, so it doesn't directly pertain to you, but I think parts of it might be helpful. So if you will allow me, I will paste it here :
KEEP IN MIND THIS IS A FILE BELOW... SO IT DOESN'T DIRECTLY RELATE TO YOUR CASE.... BUT SOME PARTS MIGHT HELP YOU .... SO I HAVE PASTED THIS INFORMATION HERE ....
Cats can often develop fears of people that are new to a house for many reasons. As I have stated if a cat is friendly and comfortable at home, but freaks out when visitors are around it often relates to not having enough experience with visitors when the cat was very young. OR, sometimes the cat may have had a bad experience with a visitor when it was young. Other times, visitors are fine until the one time somebody comes over the cat doesn't know, and there is a storm outside, lightning strikes, thunder rolls the cat is terrified and they just know it was the visitor that caused that terrible sound. Cats are strange creatures. We will probably never know exactly what goes on inside their heads. Even if we did, it might not make sense to us.
BUT, for the ones that are not medical -
HOW TO RESOLVE THE PROBLEM-
1. prevention is the best - too late here for adult cats, but next time be sure to get the kitten exposed to as many different people as possible (color, wheelchairs, hats, glasses, beards) and reward your kitten when it approaches these people and intereact appropriately
ADULT CATS -
2. the treat and reward system for approching visitors and boyfriends works for adults too, and the following are things you can try
3. Have guests let the cat approach instead of approaching them - just freaks them out more
4. have the guest squat down low and sit on the floor - this makes them look smaller to the pet and less scary
5. avoid direct eye contact
6. you or someone your cat loves should give treats and encouragement with the visitor far away as needed to make the pet comfortable - play is ok too
7. cat won't come out of hiding? try opening a can of it's food or rattling a food bag to coak it out.
8. closing doors BEFORE the visitor arrives is a good idea so it can't hide in another room
9. if your cat can learn to remain calm enough to eat or play while remaining at a safe distance from visitors, gradually entice them to come closer with other special treats or toys. This might take quite awhile to build up the trust - may not be overnight
10. if your cat begins to come close enough, 10 feet or so, have the visitor gently toss a toy or treat near the cat, but not AT it obviously. A toy to the head is not a good way to make friends here
11. Cats don't have the longest attention spans. If your cat loses interest and walks away - leave it and try again later
Anyway, I hope some of that is useful and I have given you some ideas. Good luck with this problem. And if all else fails, remember, that some cats just want to be left alone. A private room, a hinding place in a cat tree or closet, food, shelter, water, and warmth and kindness and that cat might just be extremely happy living it's life in solitude!