A "textbook" Decca tree requires omni's with diffraction spheres, and these are hard to find. (And even harder to pay for!) So I'm surprised noone has mentioned the new ones from Blue and Red. Blue OmniMouse Red R4 capsule
FWIW, I've done a fair amount of recording with 3 microphone trees in recent years, but I tend not to use "classic" Decca ones. For my money, those don't really seem to image right. There are a number of recent AES papers by folks like Theile and Williams that show you how to build 3-mic arrays with proper imaging. These are usually done with directional mics, not omnis, because otherwise you get conflicting localization cues due to crosstalk.
Here is a very useful tool. Image Assistant 2
If you have the stomach for academic papers, I highly recommend the following: Theile: Multichannel natural music recording based on psychoacoustic principles
I did some work last year using MKH-800's and was simply floored by the sound of these mics. (Must save more money...Must save more money!) Lately, I've been using OCT rigs with Schoeps 2 MK41V & 1 MK4. (These comprise a single-purpose OCT rig. With the MKH-800's you can build Williams-style arrays as well.)
Note that I've been doing LCR work -- for stereo, I would consider OCT2 instead. That requires a 1 millisecond timing shift in post. Details here: OCT2 info
David L. Rick
Seventh String Recording