Funtional role of op amps in a ADC filter circuit
Usually Vin- is connected to output Vref for unity gain and current limit but load regulated errors corrected.

1) is a standard Vcc/2 reference for single supply OpAmps using bipolar signals.

So you can call this as an "AC gnd" if you AC couple the outputs or offset after ADC = 0V with bipolar levels.  "Gnd" can be anything you decide as 0V, even if floating above earth ground.

2) is a standard precision rectifier which reduces the diode drop by the open loop gain (to 0)

3) is a standard balanced differential amp with unity gain


When \$\dfrac{R_f}{R_1}=\dfrac{R_g}{R_2}= \dfrac{V_{out}}{(V_2-V_1)} ~~, ~~  V_{CM12}{_{gain}} \$=0, gain =  ideally , actual = CMRR - 20 log R tol.