Finding changes in one branch not in two other branch

Hi,

I am not sure whether it is available. What i am looking for is

git diff <commit1> <commit2> <commit3>

And it should show changes only in in tree represented by <commit1> but not
in both <commit2> and <commit3>

To show a workflow:

kernel releases 2.6.20 and the project release patchset on top of 2.6.20
kernel release 2.6.21 and project release patchset on top of 2.6.21

The second release involve some bug fixes for the project.

The intention is to find out  what changed in project with the release
of 2.6.21 patchset
. The easiest is look at diff of diff. But if git can do it for me why not ??

-aneesh