I am a Nepali citizen and I have completed a 3 year bachelors from my country. I am planning my masters degree in Germany. Can I directly start my masters or do I have to do some courses?
Schools in Germany typically have an "International Office" or some other similar organization whose responsibilities include the "recognition" of foreign degrees earned outside of Bologna Process countries. If they approve of your bacehlor's degree as functionally equivalent to theirs, you should be fine. They may also ask you to take some additional number of classes to complete the equivalency. However, if too many credits are needed, they may refuse the request.
Good question! The Bachelor degree in Germany is also three years, as far as I know, so I see no reason for a problem.
Best way to proceed is to write to a professor you want to work with, and ask him.
Be sure when you write to mention the project title for your "bachelor thesis". This is a six-month research project carried out by German students intending to study for a Masters. If you did any kind of big research project in your third year, for which you wrote some kind of report (anything around ten pages or more) you can quote that as equivalent to the Bachelor thesis.
Good luck :-)