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.