Beyond Compare

Beyond Compare is neither free as in freedom nor free as in beer. Only the "Pro" edition seems usable for version control.

They do have a generous free trial. It's "30 days", but this only counts the days you actively used the program.

Here are some decent configs for Linux.

[merge-tools.bc] 
program = "bcompare" # On MacOS, this should be "bcomp".
edit-args = ["$left", "$right", "-ro1"]
merge-args = ["$left", "$right", "$base", "$output", "-automerge", "-reviewconflicts"]

[merge-tools.bc-3]
# Alternative config similar to `meld-3`. See
# https://martinvonz.github.io/jj/stable/config/#experimental-3-pane-diff-editing
program="bcompare"  # On MacOS, this should be "bcomp".
edit-args = ["$left", "$right",  "-mergeoutput=$output", "-ro1", "-ro2"]
# Optionally, can copy `merge-args` from the above example

For MacOS, see the comment: you need to use bcomp instead of bcompare.

I have not tried this on Windows, you will likely need to enter the full path to BComp.exe, and you might need to change -blah options to /blah.