if you know from the start how many point in each set just do something like:
a2 = randn(1,num_points_in_a2);
a = a2(1:min(num_points_in_a,numel(a2)));
a3 = a2((1+num_points_in_a):end);
If you dont know how many point you want beforehand just create and throw away the excess points you need - if it is as few as a couple of 100 elements bothering about avoiding might take more time than will be lost...