Add to your user .gitconfig
file (use tabs for indents). Paths are for MsysGit, and may not work in Cygwin:
# Default tools
[merge]
tool = bc3
[diff]
tool = bc3
# Beyond Compare
[difftool "bc3"]
cmd = "\"c:/program files (x86)/beyond compare 3/bcomp.exe\" \"$LOCAL\" \"$REMOTE\""
[mergetool "bc3"]
cmd = "\"c:/program files (x86)/beyond compare 3/bcomp.exe\" \"$LOCAL\" \"$REMOTE\" \"$BASE\" \"$MERGED\""
# DiffMerge
[difftool "dm"]
cmd = C:/Program\\ Files/SourceGear/Common/DiffMerge/sgdm.exe \"$LOCAL\" \"$REMOTE\"
Diff
C:\Program Files\SourceGear\Common\DiffMerge\sgdm.exe /t1=%bname /t2=%yname %base %mine
Diff
"C:\Program Files (x86)\Beyond Compare 3\BComp.exe" %base %mine /title1=%bname /title2=%yname /leftreadonly
Merge
"C:\Program Files (x86)\Beyond Compare 3\BComp.exe" %mine %theirs %base %merged /title1=%yname /title2=%tname /title3=%bname /title4=%mname