^ Hey there checht. Precise forward spacing of the center position is probably less important with wide omnis than a more near-spaced directional mic triplet, but between 6" and 9" forward seems to be optimal depending on the L/R spacing.
That's based on playing around with Image Assistant, which is able to model imaging across 3 mic arrangements, and also to some extent the Michael Williams multichannel array designs (the Stereo Zoom guy).
Alright, this is probably a good opportunity to let you all take a look at something I was working on last summer, but don't feel is really ready for full release yet, which I lost steam on and haven't really revisited since. Attached is a PDF which covers the extension of the Improved PAS Table from 2 to 3-microphone positions. A stereo arrangement of 3 positions will consist of an L/R pair of mics with either a single mic, or a coincident pair in the center. The basic idea is the same as with 2 ch improved PAS. The wide pair is pointed directly at the PA speakers, and the ideal spacing between that pair (in regards to imaging, Stereo Zoom like) is determined by the PAS angle. The difference in comparison to 2-channel Improved PAS is that there is now a microphone position in the center interacting with the other two, which changes things. Its OMT3 or OMT4, and because of that is going to be applicable to more tapers than any of the higher channel count OMTs.
The other difference is that to make it easier to read, rather than providing a list of angles and spacings in a table, I drew this up more like an OMT PDF using diagrams showing dimensions and angles. The first two pages of the PDF are introduction and overview. The actual diagrams are all on page 3.
This really needs verification by testing before full release into the wild, but the basic idea behind it is sound. At wider PAS angles corresponding to more narrow L/R spacings, I suspect it will work better with more highly directional mics than omnis. Take the actual spacing dimensions as preliminary for now. If anyone tries this out please let me know how it works for you!