I think you need to enable 3 way merge by default in git's configuration, and both smerge (minor mode for solving conflicts) and ediff (major mode that encompass diff and patch) will pick it up. In the case of the latter you will have 4 panes, one for version A, another for version B, a third for the result C, and the last is the common ancestor of A and B.

Addendum: I've since long disabled it. A and B changes are enough for me, especially as I rebase instead of merging.