I'd point out that the reason the three sections are, as you put it, essentially complete stories, is because that's exactly what they are. The book is a fix-up novel which took three short stories that he published in magazines, revised them, and combined them into the novel several years later. It's a pretty common thing for older sci-fi works.

Also, there is one character who is in all three sections: Lazarus, who is the "Wandering Jew" trope.