How to convert a vector to a row of a matrix and resize it at the same time in a special situation?

I want to convert a vector to a row of a matrix in a special situation which is necessary to resize it too as I explain.

for example:

vector A=[1 2 3]

should be the second row of matrix B in a special situation as shown below,as you see in the second row of the matrix B,after every two element zero we have a element of vector A.

B=[a b c d e f g h i;
0 0 1 0,0 2 0 0 3;
a1 b1 c1 d1 e1 f1 g1 h1 i1;
.. .. .. .. .. .. .. .. ..;]

My program is a large one and I need the way which is fast.

Answer 1

[Matt J]

 B(2,:)=0;
 B(2,3:3:end)=A;

[Ayob]

Yes,it works

Answer 2

[Azzi Abdelmalek]

   A=[1 2 3]
B=[5 5 5 5 5 5 5 5 5;0 0 0 0 0 0 0 0 0;4 4 4 4 4 4 4 4 4]
n=size(B,2)
id=fix(n/3);
B(2,id:id:n)=A

[Ayob]

Good but not as much Matt. method.