In Diablo 3 for the PC, PS3, and Xbox 360, you can switch between the three different followers in the game at nearly any time, you just have to go back to town to do it.
In town, the followers you have access to typically hang out to the right of your Stash.
To switch to a different follower, you just need to go up to them and click on the one you want. A dialog box should pop up, and Hire is one of the options. If you currently have a follower, they’ll stay behind and wait until you hire them again.
Even though it says Hire, it doesn’t cost money to switch followers. They can actually benefit you, since they’ll share 20% of any Gold Find, Magic Find, or Experience stat they have on their equipment.