Could three CT scans over one year all really miss symptomatic colon cancer (narrow stools, blood). Problems for past 2 years, doc says no way it wou?

Yes. Colonoscopy is not the ideal way to evaluate for colon cancer. The most accurate test is a colonoscopy. Contact a gastroenterologist to set this up.
Absolutely. Ct scan is by no means designed for detecting colon cancer. When it does, it is most often in the advanced stages since it is large enough to have either formed metastatic diseases (e.g. In the liver) or distorted the anatomy of the colon enough to become evident. The most sensitive test for colon cancer is colonoscopy. Ct is not a good test for screening for colon cancer.