I'd never given it much thought, but I believe that Romero intended them, at least loosely, to be one story. I read once that he even has an 'original intent' story that combined the three films.

I think Romero was probably so stoked just to get the money to make "Day" that he didn't waste time thinking "gee, I need to make this a period piece set ten years ago, so as not to confuse continuity."

Remember, this was the 70s. The idea of a 'film franchise' was still a ways off, and no one debated things like continuity.

(I make no claims about "Land" or "Diary" except they sucked)