I have had diarrhea for 3 days after eating a chicken burger at nations giant hamburgers , is it food poisoning?

Possibly: There is no way to tell for sure without stool cultures but it is certainly possible.

Difficult to tell: Without having the burger to test it may simply be a temporal relationship and the diarrhea may actually have been due to something else. Suggest, however, that you call the restaurant and the health department and report the association so that it can be investigated further. Hope you are better soon.