What is three-way

Two-way diff shows what changed between two versions. Three-way diff also considers the common ancestor, letting Git decide whether a region was changed by one side, the other, or both. Three-way is the foundation of merging.

Inspecting a conflict

During a merge, files with conflicts contain markers:

<<<<<<< HEAD
our version
=======
their version
>>>>>>> feature

Show all three sides, including the ancestor:

git config merge.conflictStyle diff3       # or zdiff3 (Git 2.35+)
git checkout --conflict=diff3 path/to/file

zdiff3 additionally compresses common context for cleaner output.

Index stages

The index holds three stages during conflict:

git ls-files -u
git show :1:path     # ancestor
git show :2:path     # ours
git show :3:path     # theirs

Strategies

Resolve by selecting a side:

git checkout --ours path/to/file
git checkout --theirs path/to/file
git checkout --merge path/to/file        # remerge with markers

Or by remerging with a different option:

git merge -X theirs feature
git merge -X ignore-all-space feature
git merge -X renormalize feature

Mergetool

git config merge.tool meld         # or vimdiff, kdiff3, opendiff, etc.
git mergetool

The tool opens with three or four panes (LOCAL, BASE, REMOTE, MERGED) so you can pick changes by hunk.

Recursive ancestor (ort)

When two branches share multiple merge bases (criss-cross merges), the ort strategy (default since Git 2.34) virtually merges the bases first to compute a synthesized ancestor. This dramatically reduces spurious conflicts.

Common mistakes

Assuming the ancestor in conflict markers matches your previous commit — it's the merge base, often well in the past. Resolving conflicts by deleting both sides; the marker style hides this from review. Always read the ancestor (use diff3) to understand intent. Forgetting to git add after editing a conflicting file leaves it unmerged in the index.

Replay with rerere

Once you resolve, rerere can replay the same resolution next time. See "Rerere: automatic conflict resolution reuse".

Related

See "Merge conflict resolution in depth" and "Custom merge strategies and drivers".