Mri of the lumbar but the radiologist didn't remark on a white round spot at the anterior sacrum s3 region.?

3 possibilities. There are 3 things in imaging studies that a radiologist won't mention: normal anatomy, "artifact" (false spots, lines or shadow caused by machine error/interference), and things that are simply missed because radiologists are humans. You could discuss this with the doctor who ordered the study who may be able to contact the radiologist to inquire, or seek a second opinion if still concerned.
It is. impossible to speculate on what you might be referring to without additional information or the images themselves. It might very well be a normal structure or clinically irrelevant finding(e.g. Vertebral hemangioma) that a radiologist wouldn't necessarily need to mention.