Questions about the front end design of amplifier circuit
The second circuit divides the input voltage by the resistor ratio \${Rg}\over {Rin+Rg}\$, of course, so it's fine if you want to divide the input voltage down. 

The third circuit is AC coupled. The resistors must be on the In-amp side of the capacitors otherwise they can't conduct the bias currents. 

If you analyze the circuits with a differential voltage vs. a common-mode voltage applied you can see that it is indeed different whether you use two low value resistors vs. one low value and two high value resistors. Consider 10M + 10M to ground and 1K across the inputs. The inputs are practically floating wrt ground (5M\$\Omega\$), but a differential voltage sees 1K. If you use two 500\$\Omega\$ resistors to common, then the differential voltage sees 1K, but common mode voltage sees 250\$\Omega\$ to common. Very different. 

What's better depends on the application.