So, there are two questions in this thread. They basically ask the same thing, only using two different configurations.
1) O -- C -- O
2) C -- O -- C
The 1st one, Omni, card, Omni is not a configuration I would run without spreading the omni's (ideally you would want about 1 metre of seperation). This configuration is very similar to what I run 99% of the time (I use two cards in the middle though).
The 2nd option, is the Card, Omni, Card configuration and this I run on ocassions, but it's not typical for me.
Both of these configurations produce a more well rounded sound than the typical stero card, card (or hyper, hyper) configuration. However, it is extremely important in both configurations to get a good mix/blend of the microphones. This can only be done through monitoring and making adjustments. Just last week I ran the 2nd option at a small venue for a friend of mine who was performing. I realized when I got there that I had left my earphones at home and there was no possible way I could mix, so I adjusted the dials based on about where I put them when I run my four mic configuration. Unfortunately, eyeballing the meters and dials isn't a substitute for listening and the mix is totally screwed up and sounds like crap. It does highlight the need for real monitoring regardless of what 3 mic mix you try.
Is one better than the other? I prefer to run spaced omni's with cards in between, but that's my personal preference. When done correctly either of these can produce some very sweet and enjoyable tapes. Case in point, my recording of Big Head Todd and the Monsters on Archive.org is the most downloaded of BHTM shows available and everybody really enjoys the sound quality... it's all about monitoring though, so I would highly recommend purchasing either earphones or headphones that will allow you HEAR what you are putting down on tape.
Wayne