Matrix help? Local space

If I have a hierarchy of objects like so:

bone1
--bone2
----bone3
------child

and a null object in world space, like so:

null

I have the transformation matrix for all the bones and the null, but need to put the null into bone3's local space. How would I do that? Add null.matrix+bone3.matrix or multiply them or what?

Or to be more exact, I need to find the vector by which to move the child of bone3 in local space to the position of the null.

Thanks,

Ragnar

p.s. This is not exactly Blitz related, I need to figure out the math behind this.

quick question, out of curiosity is the null object part of the original heirarchy(the parent?) or completely seperate on it's own?

The null object is in a heirarchy of it's own (it's in the world space, same as bone1). Also note that the bones can have any position, rotation and scale values.

Ragnar