1. I am using Max31865 , resistance to digital converter for interfacing PT100. (for 2 wire & 4 wire only)
2. I want to multiplex 8 rtd to it so that I don't have to use separate IC for each.
3. I have searched for analog mux like ADG708(internal resistance is 3 ohm) & DG408(RDS = 100ohm).

4.They have large internal resistance. I think these will effect the acuracy of PT100 since it have 0.4ohm/C.
5. Or they won't effect the accuracy since I am using PT100 is 4 wire configuration so drop across wires won't effect the accuracy?
6. Also for 2 wire configuration, I will use it as 4 wire only by connecting two RTD wires across RTD+ & RTD- & shorting FORCE+ with RTD+ & FORCE- with RTD-.
7. Schematic is attached. What should be best way to multiplex.
8. I think I will need 3 (8:1) multiplexer as shown in schematic. Because 4 wire of all RTD can be shorted together. Or I should use 4 (8:1) mux.

 

For best accuracy you will need to use a 4-wire connection and switch all four wires. For this you could use two 8-channel differential muxes, such as the CD4097 (one mux switches the force lines and the other switches the sense lines).
Since the sense inputs of the Max31865 are very high impedance there is negligible current through the sense lines so the wire and mux resistance will have negligible effect on the measurement.

Edit: It occurred to me that you don't have to switch the ground side of the force line if all the sensors are connected to the same ground.