Randomize visitors and write cookie for Target

I want to randomize visitors to my site so I can test 3 promotions across multiple pages. I'm thinking we randomize the visitors into groups A, B and C, and write a cookie with the value. Then I create an audience for each cookie value and serve the appropriate experiences. Using a cookie because I want return visitors to stay in the same group. But not sure how to do the randomizing cookie part. Suggestions?