Do this
A2 = A(:,3)
but note that you did not code the matrix as you wrote:
>> A = [6 2 3; 9 3 5; 4 4 7]
A =
6 2 3
9 3 5
4 4 7
to achieve the matrix A you posted in your question you need:
>> A = [6 9 4; 2 3 4; 3 5 7]
A =
6 9 4
2 3 4
3 5 7
>> A2 = A(:,3)
A2 =
4
4
7