When a radiologist two says they don't know what's causing lesions in brain and just had a new brain MRI and they third radioologist says when I asked him about a big white spot in back of brain if it could be a tumor he just said it most likely they big

See neurologist. There are many reasons for 'lesions' or 'white spots' but there shouldn't be many in someone aged 48. Would suggest seeing a neurologist for thorough evaluation and correlation between spots and clinical symptoms.