"Now you should be able to select two files in a directory in
Nautilus, right-click to bring up the context menu, and notice a
Compare entry toward the bottom of the menu. Selecting Compare will
run your preferred diff utility on these files as shown in the
"If you have only a single file highlighted and bring up the
context menu you will see "Compare later" in the context menu.
Selecting that option places the path of the selected file onto a
stack. The next time you select a single file you will see both
"Compare later" and "Compare to '/.../first-file'" in the context
"If you select two files, you will be able to compare those two
like before, but also have the option to run a three-way comparison
with a file from the "Compare later" stack. Although I mention a
compare stack, currently gdiff-ext only lets you compare with the
very last thing you added to the "Compare later" stack."