You need to be careful there, because 1/3333 is literally the division of two integers, for which the
result will be 0 ( integer ) and then assigned to 0.0 ( double ).
Some computers are smart enough to figure this out maybe, maybe the Arduino isn't.
I would suggest that you change your code to
a = 1.0 / 3333.0 ;
which should ensure a floating point division. Or just assign 0.0003 to a, if that is what you want.
Also, double on the Arduino is going to be a 32 bit standard float, so there is not much point in using it.
It is the same as the regular float, not double precision.