It's a 3x4 matrix, not a 4x4 matrix. So basically you need to fill in the last column yourself. Which is always 0,0,0,1.
It is also a global matrix so if you are trying to do a hierachy, you need to do a bit of mucking about yourself.
It is also a global matrix so if you are trying to do a hierachy, you need to do a bit of mucking about yourself.