CH3 is specified as a +/- 3.3 V analog input with up to 32x programmable gain.
However, the schematic for the input signal processing stage looks like this
*A 10MOhm pull down resistor is also present on CH3, and is not shown
This functions as a unity buffer if Rg is left floating, and CH3 is calibrated under such circumstances. But if the user isn't satisfied with the +/-100mV range at 32x PGA, and wishes to obtain an arbitrarily high gain(subject to noise considerations) , then they may ground Rg via an externally plugged in resistor. The gain will now turn into the familiar formula for a non-inverting amplifier (1+Rf/Rg . Where Rf=10K , Rg=user configured)
HB100 Doppler radar output monitoring
Among many other uses, this functionality was used to add another 10x of gain to the 32x internal gain in order to monitor the output of an HB100 doppler radar. The output of this radar is an analog signal whose frequency is a function of the velocity of the reflecting object.
Here's a video of the test