How big a plot to feed 3?

That's because your question is a bit like how long is a piece of string.  It depends on your knowledge as a gardener, what your 3 likes to eat, how much time you have available to spend in tending a plot, the state of the plot when you get it (good, bad, unloved, in good state ...) where you live (different areas have different growing seasons), how much time you are prepared to put in to getting a plot going if it's a bit rough (time invested at the start can pay dividends over the years) and whether you have your name down on a list for an allotment.   

If you aren't on the list you won't be able to take a choice of size of plot. 

So tell us a bit more so that we can help you.