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 use the program.

Beyond Compare can substitute for meld for 3-pane diff editing. This is helpful for recent MacOS versions, where meld does not currently work well.

Here are some decent configs for Linux.

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

[merge-tools.bc-3]
# Alternative config similar to `meld-3`.
program="bcompare"  # On MacOS, this should be "bcomp".
edit-args = ["$left", "$right",  "-expandall", "-mergeoutput=$right", "-ro1", "-ro2"]
# Optionally, can copy `merge-args` from the above example

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

On Windows, the configuration should be similar but use /blah instead of -blah for options:

[merge-tools.bc]
program = "bcomp"  # You may need to provide a full path to BComp.exe
edit-args = ["$left", "$right", "/leftreadonly", "/expandall"]
merge-args = ["$left", "$right", "$base", "$output", "/automerge", "/reviewconflicts"]

[merge-tools.bc-3]
program = "bcomp"  # You may need to provide a full path to BComp.exe
edit-args = ["$left", "$right", "/expandall", "/mergeoutput=$right", "/leftreadonly", "/rightreadonly"]