We all agree that Low German is a distinct language from Standard German. But what about the dialects of Central German and Upper German?

Let's suppose we find three monolingual Germans: one who speaks only their local dialect of Central German, one who speaks only a dialect of Upper German, and finally one who speaks only Standard German. Would they be able to understand each other in the same way that a Croat, a Serb, and a Bosniak can communicate? So that Central German, Upper German, and Standard German are linguistically dialects of the same language rather than separate languages?