Would symptomatic colon cancer be missed on 3 separate ct scans? i am only 37 but have had narrow stools and small amounts of blood for past year.

Unlikely : Best test would be a colonoscopy for your symptoms or at least a sigmoidoscopy. Usually if a colon cancer had symptoms a year ago those symptoms would have progressed over the ensuing 12 months and by now it would have been obvious by your symptoms and ct scan.

Possibly: If colorectal cancer was causing symptoms, it should be decently large, but no test is 100% and there are false negatives. Another screening test could confirm the ct findings like a colonoscopy.