xml question regarding document comparing

["S.Watts" <denniswatts2000@home.com>]

I have 3 documents from different companies which are their versions
of a legal document or rule.  I also have this official rule from a
governing body which needs to reflect the changes each company has put
forward.  I would like to turn each version into a xml document so I
can realize the differences each version has from the official rule. 
So I would like to create a master xml and compare it with one of the
versions.  After the two are compared the master xml needs to contain
the changes that were made.  I would like to create something that
would accomplish this rather than purchase something that would do
this for me.  I would appreciate any help you could offer that would
help me accomplish this, thanks.
dennis watts
denniswatts2000@home.com