I have a scenario where I need to show a different page to a user for the same url based on a probability distribution,

so for e.g. for 3 pages the distribution might be

page 1 - 30% of all users
page 2 - 50% of all users
page 3 - 20% of all users

When deciding what page to load for a given user, what technique can I use to ensure that the overall distribution matches the above?

I am thinking I need a way to choose an object at "random" from a set X { x1, x2....xn } except that instead of all objects being equally likely the probability of an object being selected is defined beforehand.