I adapted your 2D collision applet to check what happens, when just two balls collide like in the case I provided (one of the staying balls missing). Now I see, that my balls act differently (does it sound weird?). I must have made a mistake in my calculations (for better understanding I made my own calculations to get your result, and in one place I didn't get your way in calculating the velocities), or putting them into the code. So I will try to fix it (In the worst case I will just copy your code, am i allowed to?), and if it still give no result, I will come back with more questions.

Thank you for your efforts to help me. Now I see I can't calculate it this way (too many possible situation without just one general algorithm), but simulating single collision should work. It is amazing, that I couldn't find help anywhere, and here I got answers quickly, with clear explanation to my problem. After learning a bit physics it gets more interesting than it looked. Thank you very much!