I don't see why you need 3 vertex's to change the angles, you would want to align the weapon origin to a single vertex position of the entity(like a vertex in it's right hand) and also update the weapons angles according to the entity's angles. All that's left to do is add some offsets to make it look correct if you are in 3rd person view.

If you look at the Custom Panel in the post, you will see I have offsets for x,y,z and pan, tilt, roll. You can adjust these offsets to "fine tune" the position and angle to the vertex.

I may be totally missing what your asking for, but check out the demo and see if that works for you.

-raiden