How does someone who is really good at math choose between going into CS, Econ, or Physics?

I faced the exact situation four years ago. I had an admit for Maths and an admit for Bachelor's degree in Computer Science(both in good universities in my country).  And then, I had to choose. I chose Computer Science because I was always fascinated by computers even as a kid. And then, I realized Maths and Computer Science were related very closely than I ever imagined. I attended online lectures for subjects in Maths. The keen interest I had in Maths helped understand the complex algorithms better. I could propose better solutions than the ones existed . I could understand computer science better.I have a degree from http://coursera.org too.  Then, came the economics part. I honestly wanted to understand what the whole fuss about MBA was. My college offered courses on non-technical stuff . So, I studied four courses on economics. I was part of the team which ran an entrepreneurship lab in my college. I got hands-on experience on how to run a real-time business. And I ran one for a year. I was never really good at Physics and I do not know about that part. I think that if you have the passion, you will find a way to link all the three together. This is my story. Hope it helps. :)

Simple. Both economics and computer science are primarily focused on making money or improving the human condition (by extending our ability to gather and analyze data, and to control our environment through computerized technologies). Physics can also generate money, of course, when it is turned into technologies. For the same reason, it can also give us greater control over our environment and extend our human capabilities. But physics also has another goal (and, to my mind, a far greater and more noble goal), which economics and computer science do not, and that is to increase our understanding of the physical world and our place in it, at a fundamental level. And this enhanced understanding broadens and deepens our intellectual landscape and enriches human culture itself [1]. So the question is - do you just want to make money (for you or someone else), or develop a new mobile app or digital gadget, or do you want to be part of the greatest, most heroic cultural enterprise on which the human race has ever embarked, and which has the power to transform the collective consciousness of our species? Your choice (but it's Hobson's choice really...) :o) [1] I am in complete agreement with the late, great mathematician,  polymath and historian of science, Jacob Bronowski, when in his classic  documentary series 'The Ascent of Man', made by the BBC in 1973, he  stated that physics is the greatest cultural achievement of the human  race - bar none.

If you go into economics you will find that the research environment at most universities isn't very math oriented, whereas math as a big thing at each and every theoretical ohysics department in the world. So, there are more opportunities and more people to interact with in physics, but possibly also more competition. CS departments are sometimes math oriented and sometimes not, depending on which subjects lay ground to establishing that particular CS department. Otherwise, it depends mainly on which unanswered questions you're carrying around...