Special three-way diff

Vim can diff 3 files, but that's not enough.

I'm looking for a special kind of 3-way diff, quite similar to diffing 2
files (B, C) with 3rd ancestor file (A), but in the result, the first
two files are *not* to be merged.

scenario: A = orig file, B = my version of A, C = official update of A;
now I want to change B to include the changes introduced by C, keeping
my changes.

vim can't help me with this (3-way diff shows too many useless
differences) -- does anyone know of a good tool?

The files are not under version control (yet), I still have to check out
if Mercurial can help me with this.

--
Andy

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