What are the symptoms of colonic polyps?

None. Colon polyps are generally asymptomatic which is why routine screening is recommended at age 50 (or earlier with a family history). Rarely polyps can bleed or be large enough to cause obstruction.
Often none. Polyps are generally asymptomatic, that's why we do colonoscopy for screening.
Not typically. On occasion, polyps may infarct, twist on their stalk, bleed, or secrete (villous tumors of the rectum) with resulting "diarrhea." however, most polyps & colon cancers are identified (hopefully) before symptoms develop. Early detection at a smaller polyp size means higher cure rates, lower risk of malignant transformation or invasion, & improved survival before cancer develops. Get a colonoscopy!