This configuration is another configuration I used. Typically I use this when I'm in a very small club and don't have the room to setup my 4 mic configuration. Instead I run two cards in ORTF (SR-77's), and then clamp on one omni mic just below those. The omni gets feed into both the left and right channel. The result can be stunning when mixed properly.
As long as you use cards (of some sort) and an omni, it doesn't matter if these are a mixture of small and large diaphram mics. What matters is getting the levels set properly, then mixing the proper amount of the omni mic into each channel.
There seems to be an interest in multi-microphone configurations and I would urge everybody considering it to try a few shows like this. When done right, you'll never want to go back to stereo mics again. However, and this is the big however, you will need a mixer. This can be either a 3 channel mixer or 4 channel, but you'll need a mixer. I've been using the Wendt X4 for a few years and while it's now for sale (now using a multi-channel hard drive recorder), it was the bread and butter of my rig. Shure makes a 3 & 4 channel mixer, Wendt makes a 2, 4, and 5 channel mixer, Sound Designs makes a 4 channel mixer, and there are loads of other companies making ENG mixers for field use. All of these run off batteries and most use very little power. These types of mixers are designed to be "beaten to death" as they are abused daily in the film and video industry.
Hope this helps.
Wayne