There are two types of defaults. One is for which browser is launched when called for within some other program or by some file click. The other defaults will be settings within the browser such as the start page, how to handle history, etc. Which are you referring to? You can only set one browser as the default to use when a program wants to go to the web. You pick your favorite for that purpose. Most browsers have an internal setting to select it as the default but Windows has some default settings that might do that also.
I have no idea how to set the defaults and what happens if you pick the wrong one a particular program?