Doctor treated me for a fungal infection groin and back passage area however after 3 months thinks it scabies? I've not been in contact with anyone?

Unusual. Scabies is diagnosed when a skin scraping, of the area involved, is done, and then reviewed under a microscope to look for scabies mites or eggs. If your doctor did that and saw evidence of scabies, it confirms that diagnosis. If not, it should be done if that is suspected. It is treated entirely differently than fungus. should have that rash thoroughly inspected soon.
My experience. My experience with scabies is that if it is untreated it will spread to other areas of the trunk and limbs. If it has remained local I doubt if it is scabies.