>I think you give me a good suggestion. I added reference of two project to the third one. But then, don't I have to still include somewhere the "using MyNameSpace"?

I the using directive is not really needed but it makes the code a lot easier to write and read if you use it. Using the using directive just allows you to use the namespace containing types you want to use without having to specify their fully qualified name.
The scope of using is to the file it is in so you have to add it to all cs files where you need it.

E.g., if you didn't put using System.Windows.Forms; in the top of your file you would have to write:

System.Windows.Forms.MessageBox.Show("Hello World");

instead of the much shorter:

MessageBox.Show("Hello World");

Hope this makes sense. Please let me know if it doesn't.