LCR panning is a MAJOR project. You really have a choise of custom-build, multi-gang precision potentiometers, OR seriously convolved electronics.
You will not find it here. I have the SSL LCR schematic, and it is a majorly complicated undertaking, with several op-amps, a pair of SSM2402-type electronic switches, and STILL with a custom-build 3-gang pot.
...and in order to get divergence control, you need even MORE pots and active electronics.
The Amek does it differently, and nowhere NEAR as well. That uses switvched ladders for several take-them-or-leave-them divergence settings, but it is NOT clever enough to do hard-center panning.
You seem to be dead set on passive summing; -why, I have no idea- yet you also seem to be very keen on "L-C-R panning" though you haven't clearly defined your divergence requirements... that too will be crucial.
Hell, PLAIN STEREO panning with a passive summing network is super-lossy and VERY prone to bleed-back and crosstalk.... mixing that with LCR panning is going to be:
1) ULTRA lossy
2) VERY prone to crosstalk
3) SUPER-interactive
4) a VERY bad idea.
Even the active solutions are sufficiently complicated to be moderately daunting in terms of cost to most manufacturers, so I suggest you build a 3-channel summing system with perhaps 6 hard-asignment positions:
1) left only (hard left)
2) right only (hard right)
3) Left-and-right only (fully diverged center)
4) center only (hard center)
5) left-and-center only (halfway-left)
6) Right-and center only (halfway-right)
Your options would realistically be to build something like the above, or to just buy a real LCR panning mixer...
-Seriously.
Anything else is really Fred Flintstone, very hand-to-mouth in that it may work, but isn't actually a good solution.
Keith