There are several methods. The easiest way is to use a programme like rEFIt which is a 3rd party boot menu manager. But that is NOT the best option imo cause during every boot you would need to pass through the new boot menu, which annoyed me a lot. So I made a tutorial using a slightly more complicated method, but the results are worth it as you do not require any 3rd party apps. To boot into Linux or Windows, you would just need to press alt/option key to access the bootcamp partition, as usual. If you don't want to, you can boot in mac just like normal. check it out. But before you jump into it, back up everything!! its a safe method if you are careful, but you never know.... If you have time, and are comfortable using terminal, there is a better way of triple booting your mac. Back UP before however!!