You could try:
x = false(3000000, 1350);
That will consume less memory than creating a double array of that same size, though it will still require a large chunk of memory. If you expect the array to contain only a few nonzero elements, instead I recommend creating a sparse array using either:
x = sparse(3000000, 1350, false);
or the spalloc function (if you know roughly how many nonzero elements it will contain.)
