not as easy as you'd think.. to be honest, it's boarderline impossible in most countries unless you're doing it through some sort of global staffing/locum tenens type thingy.
the places where one would probably want to work for lifestyle (europe, some nice beachy spots in South America) generally have stringent locals only policies to fill the need (of course its easier if you have ties to the country or are married to a local)..
secondly, you can't expect to practice anywhere without language proficiency and they usually make you sit exams for this..
thirdly, you have to remember that EVERY country has their own rules on residency requirements, boards, etc etc and some are very different from the US so you would again need to take exams etc. to prove that you're what they would consider competent.. being an American isn't enough

..
and then there's the fact that the US healthcare system is very different from most other places so there would be a learning curve in trying to deal with the way they do things..
But for Australia, it does seem to be the easiest of the 3 you mentioned to work in.. Most global thingies i've seen love to send US Docs there..
in short, go to medical school and do residency where you want to set up shop and practice.
if when you become a physician, you're lucky enough to have an amazing group of colleagues that wouldn't mind picking up the slack for you while you travel the globe with a locum tenens company or NGO, that would be the way to go.