Hi Sheogorath, yeah there is no difference between 2 or 3, I just don't leave them at 4 or more deep unless they are militia. I'm talking mainly about Line Infantry as well.

If there is cav around I'll keep them in 3 or perhaps 4 deep if there are heavy cav in the area.

As for cav, I've also kept them in 4 or 5 deep...they'll punch right through on occasion and that's a kill shot against most infantry units.