the best answer would depend on the amount of moisture required for
your particular breed of grass and soil and drainage, subject to any
water shortage restrictions in your area.
the short answer is try it for a week in hottest weather and watch the
lawn carefully.
irrigation can easily be more effective with additional heads, of
course. deliver water at optimum time of night for effective watering
with low evaporation.
read this:
http://www.gardening.cornell.edu/law...teringyourlawn


wrote:
So I'm wondering whether an area of grass being watered by two
sprinkler heads is enough coverage or whether it's necessary to have it
watered by three? I've designed my sprinkler layout (please see
http://www.sunriveronline.org/misc/sprinklers) but no matter how I
slice it, there are a few small areas that only receive water from two
sprinkler heads. Is this sufficient?

-Fleemo