A, B and C means three-way. C是A和B的父亲，D是A和Bmerge之后的结果

The three-way merge uses the ancestor of the changed files to identify blocks
of content that have changed in neither, one, or both of the derived versions.

使用C的内容来标记A和B的内容有没有改变（1.A和B都没有变，2.A或者B变了；3.A和B都变了）

Blocks that have changed in neither are left as they are. 如果是1的话，不需要merge。

Blocks that have changed in only one derivative use that changed version.
如果是2的话，D的内容和改变了的A或B一样。

If a block is changed in both derivatives, the changed version is used if it
has the same content on both sides,

如果是3的话，有分为两种，A和B的改动一样，则D的内容和A或者B一样；

but if the changes differ, it is marked as a conflict situation and left for
the user to resolve.

但是如果A和B的改动不一样的话，这时就会进入冲突模式，需要人去手动的解决了。

下图就是在clearcase中出现冲突后使用xmerge解决冲突的截图（相关code我已经隐去）。

http://en.wikipedia.org/wiki/Three-way_merge
http://en.wikipedia.org/wiki/Diff3
kdiff3. sourceforge.net