# meld --output o.txt l.txt b.txt r.txt
In general meld displays
l.txt (or local version in case of git, to which version merge is done) in left tab,
b.txt (or base version in case of git, common ancestor to both) in middle tab,
r.txt (or remote version in case of git, from where merge is done) in right tab.
Now make changes to b.txt and save.
Meld saves the changes to o.txt (or working copy in case of git, which is going to be commited).
Files l.txt, b.txt, r.txt all remains unchanged.