The beauty of a liberal arts degree is that you can major in something and fill the rest of your schedule with whatever interests you. There is zero reason to triple major.
Look at what the IR major entails at your school. At many, there are lots of economics classes, so you will get that exposure anyway (and load up on more if you like it). Most require FL to a relatively high level. All involve political science and history. My point is that a second major could very easily fall out of the IR major.
Many schools have distribution requirements as well. If you study abroad (great idea for IR majors), you may also be working that in.
In other words, your 4 years will fill up quickly with what you need for your major and to graduate, and a minor or second major could come out of that naturally. Adding philosophy as a major (rather than taking some classes in it) will make your life complicated and will not really add anything for someone evaluating your transcript.