Tools For Merging Diffs?

RossyB asks: "Are there any tools to assist merging 2/3-way diffs? When I work on large changes to CVS sources I copy the tree and edit it separately, but then if the CVS source is heavily changed I need to manually update my changes. Merge conflicts from CVS suck, dirdiff helps a bit, gtkcvs is okay, but what I want is a tool to show me the differences between two or three trees, and allow me to edit/copy/delete files to merge the two. Hooking into XEmacs so I had syntax- and change- highlighted source code (in two windows) would be fantastic."