Is it likely that three abdominal /pelvic ct scans taken over the course of a year (for colon issues) would miss colon cancer?

It is possible: In a routine ct, the colon can be very difficult to evaluate. Stool throughout the colon makes it difficult to identify small masses (i.e., early cancer). Also, there are almost always areas of contraction in the walls that can mimic tumors. Unless the ct is done after a thorough colon prep and with a specific technique designed to identify polyps, colon cancers can "hide" very easily.

Yes: Do you have a family history of colon cancer. If so, do not rely on a ct scan, as cancerous lesions could be missed unless you have a ct colonography, which is different from a ct scan of the abdomen and pelvis. You may also wish to undergo a routine colonoscopy if you are over the age of 50.