You can set Beyond Compare as your default diff tool by following the instructions posted here.
Diff
At a Windows command prompt, enter the commands:
git config --global diff.tool bc
git config --global difftool.bc.path "c:/Program Files (x86)/Beyond Compare 4/bcomp.exe"
Note: For Git versions older than 2.2 (git --version) replace "bc" with "bc3" in the above instructions.
3-way Merge
At a Windows command prompt, enter the commands: git config --global merge.tool bc
git config --global mergetool.bc.path "c:/Program Files (x86)/Beyond Compare 4/bcomp.exe"
Note: For Git versions older than 2.2.0 (git --version) replace "bc" with "bc3" in the above instructions.
Launching Diffs and Merges
File Diff:
git difftool filename.ext
Folder Diff:
git difftool --dir-diff
3-way Merge:
git mergetool filename.txt
Advanced Settings
To disable the "Launch 'bc3' [Y/n]?" prompt, run the command:
git config --global difftool.prompt false
Git's default settings retain merge files with *.orig extensions after a successful merge. To disable this safety feature and automatically delete *.orig files after a merge, run the command:
git config --global mergetool.keepBackup false
Running the following command will open your .gitconfig file in an editor.
git config --edit --global
You will be able to see what the command did to edit your .gitconfig file.