I am currently working on the circuit attached. However I need to use 3- wire pt-100 as the temperature sensor instead of ds1620.... Please help.

 

Hi,

To build such a controller around a pt-100 you have to re-think everything starting with:

-Pt's require a stable constant current source for them to work propperly
-Feed the output to a good low tolerance resistor to turn the current into a measurable voltage
-Feed this voltage to an ADC converting IC or MCU pin. (if ADC is supported)

As it is, I would reccomend you to stick with the DS sensor.

Good Luck.