How to perform a three-way diff in Git without merging?

I want to be able to perform this diff before merging, including on files without merge conflicts.

You just have to set up the index as you like, you never have to commit results. The way to set up for exactly what was asked, straight diffs-since-base with no merge prep, is

git merge -s ours --no-ff --no-commit $your_other_tip

a complete handroll in which git only sets up parents for whatever you eventually decide to commit as the a result, but it’s probably better to do this with a normal merge while still being able to get in there and examine everything,

git merge --no-ff --no-commit $your_other_tip

Pick your starting point, and then

1. force a merge visit for all entries that show any changes in
   either tip:

   #!/bin/sh
   
   git checkout -m .
   
   # identify paths that show changes in either tip but were automerged
   scratch=`mktemp -t`
   sort <<EOD | uniq -u >"$scratch"
   $(  # paths that show changes at either tip:
       (   git diff --name-only  ...MERGE_HEAD
           git diff --name-only  MERGE_HEAD...
       ) | sort -u )
   $(  # paths that already show a conflict:
       git ls-files -u | cut -f2- )
   EOD
   
   # un-automerge them: strip the resolved-content entry and explicitly 
   # add the base/ours/theirs content entries
   git update-index --force-remove --stdin <"$scratch"
   stage_paths_from () {
           xargs -a "$1" -d\\n git ls-tree -r $2 |
           sed "s/ [^ ]*//;s/\t/ $3\t/" |
           git update-index --index-info
   }
   stage_paths_from "$scratch" $(git merge-base @ MERGE_HEAD) 1 
   stage_paths_from "$scratch" @ 2
   stage_paths_from "$scratch" MERGE_HEAD 3
   

2. … if you were using vimdiff, step 2 would be just git mergetool. vimdiff starts from what’s in the worktree and doesn’t do its own automerge. It looks like kdiff3 wants to ignore the worktree. Anyhoo, setting it up to run without –auto doesn’t look too too hacky:

   # one-time setup:
   wip=~/libexec/my-git-mergetools
   mkdir -p "$wip"
   cp -a "$(git --exec-path)/mergetools/kdiff3" "$wip"
   sed -si 's/--auto //g' "$wip"/kdiff3
   

   and then you can

   MERGE_TOOLS_DIR=~/libexec/my-git-mergetools git mergetool
   

Backout from this is just the usual git merge --abort or git reset --hard.