You can set up your campaign so a specified percentage of visitors sees each experience.
For example, you might have three experiences, and set them up so 50% of visitors see Experience A while 25% of visitors see Experience B and another 25% see Experience C.
When a visitor enters the campaign, a random number is generated. That number is used to choose the experience to display. If there has been very little traffic, for example at the beginning of a campaign, the traffic might not be split exactly as specified in the percentages. The more traffic you get, the more exact the percentages will become.
The total of the percentages for the campaign should add up to 100%.