Ditto to what everyone else has said. Repeated heat cycles will definitely make a cat cranky, uncomfortable, and is not good for her long term health.
Unless you are breeding her.....do get her spayed. Think behavior is directly related to hormones!
She will be a much happier, healthier kitty if you get her spayed. Spayed females have a much lower rate of breast cancer!
Please consider spaying her!
Well, I'd say you've really got your hands full. She's 3 and never had kittens? Sounds like she acting frustrated, needy and agressive. I really think you'd have a much better pet on your hands if you would just get her spayed. Is there any reason why you have not done so?