I don't know what to say when meeting the supervisor after he has suggested me to leave the research group I am a PhD student in Germany and my supervisor has suggested me to change the research group because he has told me I cannot write a good quality dissertation within the time framework of three years. He assumes that the maximum possible time for doing the PhD is three years and this time framework cannot be extended.

Now I want to talk with him face to face, but I don't know what to say. Has anyone had a similar experience? Should I fight to keeping this position or I should find another position in another university?

edit 1: I personally prefer to leave the group because I never felt I'm part of the research group even after one semester. I didn't get that feeling even from my supervisor.
This adviser is doing you a favor. He is telling you at the very early stage of your PhD that you're not cut for it according to his standards. That sounds harsh and that's why I think the funding argument is merely a diplomatic excuse. 

Imagine if he had told you the same thing after 2.5 years, or failed you at the defense (if that still happens).

I have no ways of knowing whether he is right or wrong, but that is irrelevant anyway: he doesn't want you to get a PhD under his supervision, hence you won't get it. Your only viable option is to leave.

For your discussion, ask more details about what you could improve, or what other path he would suggest for you. He is in no way obligated to answer but it might be helpful to you. Based on what he tells you, you might get an idea of what can be next for you: switching group, taking additional education, switching field of research, looking for opportunities outside academia, etc. That is not something we can help with without knowing you and your work.