You don't have to use Egit here.
You can change the diff manager in CCRC in order to call any diff tool you want:
See "Changing the Diff/merge Type Manager in CCRC 7.1.1 and later"
As mentioned in "How to Use Kdiff3 as a 3-way Merge Tool With Mercurial, Git, and Tower.app":
Installing
kdiff3
is as easy as downloading the latest version from sourceforge and copying it to your/Applications
directory.The app is a simple wrapper around a Qt-based application.
It can be run from the command line at/Applications/kdiff3.app/Contents/MacOS/kdiff3
.