Usually I like to track one singer at a time or mic individually but this
group wants to sing together in the same room. Got a big beautiful
sounding
room for them but could use some micing advice. My available mics a

2 - Rode NT 2's
1 - Neumann TLM 103
2 - Neumann KM-184's
1 - Oktava MKL 2500
1 - Sennheiser MD-421

For stereo I'll put them through a Peavey VMP2, mono an Avalon 737sp.

Are they insisting on singing into a mic pair, or can they each have their
own mic?

If I were you I'd insist on the latter. The odds of three voices working
well with one mic type is pretty slim. If possible have them face each
other in a triangle, at least 6' apart to get decent separation.

If you have to use a stereo pair, it's pretty much got to be the NT-2's in
an ORTF-ish format, and it wouldn't hurt to have the KM184's further back in
XY so you can play with the room sound. I suppose it's worth trying the
KM184's up front, but the NT-2's aren't nearly as good room mics IMO. The
ORTF pair will have to be done right on site, so play with mic positioning
until you get what you want.