How high would chances be that symptomatic colon or rectal cancer be missed on 3 separate ct scans? would this be very likely?

Colo-Rectal Cancer : It wasn't clear to me whether or not you are "symptomatic." if not, and you are young <you didn't provide your age either (colon cancer is more likely to appear after age 50 unless you have a cancer gene)> chances of missing a tumor on 3 ct scans are low, but possible. A better screening test is a colonoscopy. You should discuss this with your doctor if it remains a concern.

Possible to miss: The colon has many folds and turns with stool inside it. It would have to be big enough for a ct scan to show it and contrast inside the colon might help identify it on a ct scan. But it can miss a lesion. The best way to see colon cancer is taking a prep to clean out the colon and getting an endoscopy (colonoscopy) that can visualize the cancer and biopsy it at the same time.