Run these commands for Beyond Compare 3(if the BCompare.exe path is different in your system, please replace it according to yours):
git config --global diff.tool bc3
git config --global difftool.bc3.cmd "\"c:/program files (x86)/beyond compare 3/BCompare.exe\" \"$LOCAL\" \"$REMOTE\""
git config --global difftool.prompt false
Then use git difftool