How do I install Linux on a Mac that already has XP via Boot Camp?

So I've installed XP already on a unibody MacBook Pro and would like to have Ubuntu as well. I am wondering if it's possible to add a third operating system, Linux/Ubuntu. Is it possible or am I going to have to get an external hard drive to install Ubuntu? I'm thinking about just putting XP on an external hard drive and install Ubuntu via BootCamp instead… Help? Please??

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