Prostitution? But even that isn't going to hit $2.5 mil in three years. More likely, you'll need to make all 2.5 mil in your third year, and spend the first two building yourself up.
You could try doing some tricky loophole pyramid scheme stuff using cryptocurrencies like Bitcoin? But that's also pretty scummy.
I don't think you can get more than 100k for one, and even that sounds high. You'd have to find 25 people with two working kidneys, which is almost definitely going to take more than three years (and a lot of ice).
I am currently studying computer science and multimedia, I can currently in program c++, html, java, action script.
I am also quite fit. Have experience in gym, squash, personal training.