Do I have enough and thick donor area for FUE transplant?

Most people have good donor hair but it generally requires a microscopic exam to make sure there isn't any miniaturization.

The cause of the hair loss is just as important as the donor availability. The reason they don't recommend hair transplant may be several factors.  Contact the doctors you visited and ask exactly why they don't recommend it. 

You have already seen three doctors who said that it was not good enough. That might mean that you have a condition called Diffuse Unpatterned Alopecia (DUPA) which is a contraindication for a hair transplant. Ask the doctor why they though you are not a good hair transplant candidate.