I agree with Chris. It applies to both situations.
Gunnar was right on the fact that it applies to seperate sources, but was wrong about it not applying to stereo micing.
Hmm.
Teddy, seriously, how often do you use this 3:1 rule in a stereo pair? Could you make a drawing of how the two mics are setup in relation to the single source (which might be smallish like a guitar or large like a symphony orchestra). Let us together once and for all kill the idea the 3:1 has anything to do with the two mics in a single stereo pair.
Anyway, the spaced stereo technique of two omnis is one of my favourites. It is sometimes called AB. Depending on the room, the mics may be about a foot apart or up to many feet, maybe as much as 20 feet in extreme situations (far from the sound source).
Anyone experimenting should try this. As the distance increases you often end up in a situation with a "hole" in the middle of the sound stage. This is clearly heard on a good pair of monitors, but sometimes not noticed in headphones. When the hole is there, you know you are too far apart. On the other hand, as the mics get closer to each other, the stereo width decrease, finally collapsing into mono. When the mics are really close you may end up with some quite interesting phasing phenomena and wandering stereo images (one sound, say the S-sound of a chorist may wander from left to right in the stereo image). Somewhere in between too close and too far apart is the optimum. Sadly enough there are no rules really, you need to listen to get the right distance. In a hurry I tend to go for about 2 foot, usually works.
Gunnar