insert 0 in the each 3N location

I have matrix A that has something like this

 A = [ 1 2 .5 .6 -.7 .9, so on]

Now I want A to fill with zero at location 3, 6, 9 to 3N.

 A= [ 1 2 0 .5 .6 0, -7, .9 0 , etc.]

Answer 1

[Laura]

 B=zeros(1,3*N)

 for i=1:N
     B(1,3*i-2) = A(1,2*i-1);
     B(1,3*i-1) = A(1,2*i);
 end