Right side EGT normal then drops at idle

Is it real or is it an instrumentation problem? It sounds like the latter if it is dropping to a very low value. Is the engine running properly or is it rough? Can you easily swap the temp probes and wiring over and see if the fault moves or stays on #3?

Sounds like either a bad sender or a bad wire connection. I'm not sure it's the gauge because it corrects itself by adding rpm. It's possible. I'd check the wiring good first  (connections and ground) and then for $45 replace that sender.

I adjusted the idle mixture screw, idle stop, and synchronized the carburetors. They are right on according to both the Carbmate and analog needles. I have seen a lot of problem sensors, but it's usually obvious. I can see about switching them to check if it's the sensor.

Swap the two EGT wires and see if the problem stays on that one EGT.

Will be trying that today.