It's already stickied at the top of the active Microphones & Setup
(this thread is in the knowledge base / archive sub-section)
There have been numberous discussions of it here at TS over the years. A search will probably find a number of them, although some are burried in threads that do not have Stereo Zoom
in the title I'm sure.
Michael William's website should be linked- http://www.mmad.info/SOS_MMAD.htm
The link there to microphone array design
leads to a hyperlink-based interactive selector for choosing Stereo Zoom based configurations. Run through it and download or print a PDF showing angles and spacing for your choice of mic pattern and SRA. He has extended the Stereo Zoom to include not only two channel but up to seven channel microphone arrays. Just pick how many microphone channels you want and follow the links. Most here will simply use the 2-channel arrays, but the three (and four) channel arrays should be useful as well. The straight three channel pages are not active, but you can determine three channel configs by running through the 5 channel links and simply ignoring the 2 surround channels. Although designed for playback with the same number of speakers as mic channels, I've found that these mix down very well to stereo without problems, so they are good safe choices for 3 & 4 mic setups from one location with the intent of mixing to 2 channel, rather than just combining two seperate stereo pairs or guessing on an appropriate 3-mic setup.
I've personally found the site very useful for choosing between various appropriate 5 channel surround recording configurations, where the extention of the Stereo Zoom beyond just two mics gets complex. I'll print several candidate configs out and choose between them.
All his papers may be downloaded at no charge (and are excellent reads I recommend for those interested) here, which is a page not linked from the home page- http://www.mmad.info/