Consider a 4-channel recorder for running 3-microphone configs. That will allow you to tweak the resulting recording to perfection by mixing the 3 channels afterwards by ear, instead of having to mix down to 2-channels live using the Nak mixer by guess.
Four channels also comes in handy for recording 'standard' two channel configurations plus a soundboard feed if available, or for a running a second two-channel setup for comparison purposes.
Kudos for your mention of running a 3-mic configuration. I've suggested using a single center shotgun or supercardioid between spaced omnis (or in this case cardioids) for years. And it leaves one channel of a 4-channel recorder free for SBD if available.
Just let me know if you'd like some suggestions for optimizing that 3-mic configuration you mention for specific scenarios. What you describe turns the old 3-mic Nak technique inside-out (one shotgun between two spaced mics, as opposed to two shotguns with an omni between them in the center) and is a variant on the multichannel technique I prefer. As a general guideline, you'll want to use more spacing between the two cardioids than you probably think you'd need - at least twice as much as you would use between them if not using a center microphone. Depending on the spacing you'll also probably want more angle between them as well. If constrained to using a standard-length short stereo mic-bar, don't hesitate to point the cardioids fully 180 degrees apart, directly away from each other, facing to either side. As the spacing between them is increased, begin to narrow that angle. At the opposite extreme with all three mics facing directly forward, a spacing of 5' or more between the two outer cardioids is appropriate.