The big question: what do you want to do with your life? There are some VERY different choices with those three.
PhD: a choice within a choice: academia versus corporate. Academia has the benefits of excellent job security, (usually) good benefits, and (usually) interesting work. Has the detractors of semi-low pay (in comparison) and putting up with the BS of academia. (I actually AM a PhD student in Engineering Mechanics). Corporate research pros: GOOD pay, interesting work (sometimes). Cons: a lot of BS (be prepared to hear TQM or "six sigma" over and over again), next-to-nil job security in today's economy.
MD: pros: interesting work (depending on what you do), VERY good pay (again, depending), having people address you as "Dr." all the time (surprisingly enough, that doesn't happen with a PhD out in "real life"--just at the university). cons: takes QUITE awhile to earn money (school, residency, etc), malpractice insurance is VERY costly, unless you're in private practice, be prepared to work a LOT.
JD: pros: patent (biomedical, particularly) law can be VERY profitable, and you get to hear about all the pharmaceutical lawsuits, pay is good once you pass certain barriers. cons: the first few years are tough (you work long long hours to get to partner and join "the boys club") and you're a LAWYER (not all of them are bad--some are great--but you DO get that stigma from the ambulance chasers).
So its really what you want to do. What fits in with your plan for your life. Personally, I chose a PhD (instead of an MD/PhD program at Duke) because I like research... and I particularly want to have a family. And professors at research I universities (especially tenured) have excellent job security, good benefits, and can likely stay where they are for 20 years (which is good for raising children). HOWEVER, this is not the right choice for everyone.... you need to think what YOU want and what your priorities are. Good luck!