Diff3 is already installed on Ricky.
# diff3 --version
diff3 (GNU diffutils) 3.3
Copyright (C) 2013 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>.
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.
Written by Randy Smith.
# which diff3
/usr/bin/diff3
You probably just need to edit LocalSettings.php and set
$wgDiff3 = "/usr/bin/diff3";