The third one is a bad idea, since files in the labkeyWebapp may get overwritten when you upgrade. But don't despair there are even more options:

create a directory called extraWebapp next to labkeyWebap and put the file there.

Or...

create a mini-module and put your css file in the webapp directory.

I'd recommend the /home/@files method, or if you already have a mini-module that you're installing on your servers, put it in there. There's a big advantage to having the file path not change between say development and production machines.