Okay, if you need both new and return visitors under the same activity for some reason you can use an XT activity to achieve this. Here's how I'd set it up:
1. Create a profile script to randomly assign visitors a number between 1-100 (this will be reusable for future tests too). In my example name it randVisitorNum. Only assign a number to a visitor if it hasn't already been assigned (you don't want the visitor's number to change each page view). Here's what the script should look like:
if (!user.get('randVisitorNum')) {
return String(Math.floor(Math.random()*100+1));
}
2. Then create an audience for people from 1-50 and a second audience for people from 51-100. Like so:

3. Setup your XT activity. With a the "new visitor" audience on Exp A. Then use a "combined audience" for Exp B (return visitor + mutually exclusive Audience A (1-50)), and similar for Exp C.
That should get you want you are trying to achieve. Let me know how it works.