Thank you for the query.
Blood on the surface of the stool is most likely caused by hemorrhoids. It takes about 10 years for colon cancer
to develop, so your colonoscopy
3 years ago is still valid.
The other thing is that doctor is unable to tell about hemorrhoids condition with rectal examination
as most of hemorrhoids are hidden inside the rectum
So I suggest you to have rectoscopy done and consider surgery.
In a meanwhile please try Oak Bark sitz baths and hemorrhoids suppositories.
Hope this will help.