I know all dogs are different, but I have never known a female dog who threw up 3 times a day as a matter of course, just because she was coming on heat. I have owned two 'entire' female dogs in my life and have known people who also had 'entire' females, and never saw this.
I think if your dog has been vomiting three times a day for six weeks.....it is definitely time to get a vet's advice. She needs to see the vet and get this properly checked out.
Call up your Vet and have a chat,!!! 6 weeks is far too long to leave it.It could be many things apart from coming into heat.For instance allergies and wheat intolerance.