Here's some basics I've discovered using setups with three main microphone positions (which could either be 3 mics, or 4 mics if using an X/Y or M/S pair in the center)-
Conceptually (and in practice) I start from three omnis. Three omnis are probably the most flexible arrangement of any in terms of spacing, as long as the room supports using omnis, which typically means outdoors unless setup on-stage or at the stage-lip.
An appropriate spacing between three microphone positions is much easier to achieve than the most appropriate spacing between two. Partly because of the ability to fine tune the level of the center separately from the sides, and the ability to make that adjustment completely separately from the left/right balance. Using an X/Y or M/S pair in the center makes that even less problematic, as that provides control over the stereo width and image blend of the center portion, as well as control over center level.
I generally shoot for a spacing between left/right omnis of about 1 meter minimum. Using an X/Y or M/S pair in the center instead of a single mic in the center can help wider spacings work well. I'm less hesitant to go over 2m wide with the Left/Right omnis if I'm using either a center omni or a center coincident pair.
The first place I substitute a directional microphone for an omni is in the center. That's because the center is where I want upfront focused clarity in the playback image. The L/R omnis extend the audience reaction, room ambiance, and stereophonic width outwards to the sides from that solid, clear center. The further back the recording position is located, the more forwardly directional I'd like the center microphone to be. These days I always use either a cardioid or supercardioid in the center, because even on-stage or at the stage-lip I want to attenuate the room ambiance and audience reaction from the center playback image as much as possible. That keeps that valuable ambient stuff from competing for focus with the musical material, and allows for a better balance with more of it in the finished recording. In any case, I'd almost always rather use three omnis than just two.
I never hesitate to turn that center directional microphone into a Mid/Side pair, as long as I have an extra recording channel and a figure-8 mic available. If I had an small M/S or X/Y stereo mic I might use that in the center.
I prefer to use omnis as L/R when possible, but when recording indoors, I may want to switch the L/R mics to a directional pattern in an effort to push the direct/reverberant pickup ratio towards favoring whatever direct sound is available. Combined with sufficient angle between the L/R mics, this change can also allow for less spacing between the L/R pair, making the setup more compact and easier to rig (even though a 3 channel setup is almost always going to be significantly wider than a 2 channel stereo setup).
Using directional L/R mics, an appropriate balance needs to be found between L/R spacing and L/R angle. If using less spacing between mics than one would for omnis, one needs to introduce more angle between microphones. If you want to point the L/R mics directly at the stacks, you need pretty much the same spacing as using omnis, which might be appropriate if recording from the back of the room.
I did a medium sized club date for Karl Denson last month, using supercards as the L/R pair and a cardioid plus an bidirectional in the center. I was next to the SBD, centered and about halfway back in the room. I spaced the L/R supercards around 30" apart (the maximum my re-purposed mic bar would allow), with a 90 degree angle between them. Each pointed 45 degrees off center, which was maybe 10 to 15 degrees outside of the stacks. I would not want them angled any less with that limited spacing, otherwise the center mic and the L/R mics would be too close and not angled enough apart from each other. We need sufficient difference information between each of the microphone positions, and that's achieved only via spacing and/or angle.
Along those lines, it can be helpful to consider your 3 mic setup as the combination of two separate 2-channel stereo setups. In my Karl Denson example, the left supercard and center cardioid formed a 2-channel pair with a spacing of about 15" and an angle between them of 45 degrees. Likewise for the right supercard and center card. That doesn't seem like very much mic angle for that spacing and it isn't (compare to DIN at 12"/90-degrees). I would have used a bit more spacing, but having maxed out what I had available, I angled the mics a bit more than the outside edge of POS.
An optional fig-8 making the center microphone into M/S pair is gravy. Ignore it when figuring the spacing and angle of the L/R mics. At mixdown time, treat the center microhone as a single center mic (all Mid, no Side) when finding the most appropriate balance between Center and Left/Right. Once you've dialed that in to your liking (balance the Left/Right pair in isolation before introducing Center), play with the Mid/Side to widen the center a bit while listening for the smoothest blend between the center and the left/right edges. If using an X/Y pair in the center, you can do the same by making a Mid/Side stereo-width adjustment on the X/Y pair. Start with all Mid, then dial in the center width to taste. If that's too much hassle, just bring up the X/Y pair in tandem to fill the center at mixdown, and maybe err towards using less angle rather than more between mics in your center X/Y setup. 90 degrees is probably okay.
It is advantageous to think of your microphone setup and your mix-down choices in terms of stereo-center and stereo-width more than in terms of left/right-stereo. In terms of Left/Right, pretty much all we can do is tweak things to get the balance centered appropriately. In choosing the most appropriate balance between center and sides, between direct and diffuse, between narrow and wide, and the blend between those things, we are presented with far more flexible aesthetic choices.
The third (or 4th) mic gives us freedom!