I'm going to disagree with this because I think, no matter how many people you have in your shot, crossing the line of action will only confuse your viewers more. Unless you show a player moving to a new position with a master shot, player moves or camera moves or both move, then you MUST re-establish your master shot. Crossing the line of action just because you have dialog between three players is not a good idea.technically speaking, the 180 degree rule confuses things when dealing with a scene that has 3 or more characters.. because the line can and will be crossed whenever a character turns to look at another in the scene..
Here's my case in point. In the movie "Chocolat"; Juliette Binoche, Judi Dench and Lena Olin are in the typical A formation for three person dialog. Binoche and Dench are almost perfect 45 degree angles toward each other. The master shot captures all three women. Olin is behind the counter, binoche and Dench are seated at the counter but turned half way toward each other. The master shot captures all three women, then dollies in on Dench as she begins the scene. Never once does the camera cross the line of action so wonderfully established by the counter.
My suggestion is that you should never cross the line of action without two things; A good reason and another establishing master shot.
Granted I'm new to this whole directing thing but I thought this was a staple?