A PT100 is a temperature sensor called properly a RTD (resistance temperature detector), is simply a resistor that varies it's resistance in a very specific manner Vs the temperature it is exposed to. They are very precision sensors capable of very accurate measurements. Two of the wire represents the variable resistance and the third wire is attached to one of the other wires at the sensor so that in the proper circuit the length of the lead wire back to the circuitry will be able to factor out the fixed resistance that the wire distance represents. A RTD is not used directly by itself but is rather the variable component in a typical wheatstone bridge configuration, usually followed with a instrumentation quality op-amp stage to raise the measurement voltage to a more useful value. There are also special purpose ICs designed to interface with RTD sensors to make the conversion from variable resistance to variable measurement voltage a little simpler to realize.
So we really need to know how you are wiring the RTD up to see if you are close to having a viable solution. in a wiring schematic. It would require at the minimum a second fixed resistor so that you could form a voltage divider so that the changing RTD resistance would be able to be measured as a changing voltage capable of being measured by an arduino analog input pin, but as I said the vast majority of usage of RTDs is with a full wheatstone bridge configuration followed with amplification.
Here is a handle PT100 temp Vs resistance and resistance Vs temp calculator so you can see the range of resistance change that any given temperature range will result in.
http://crivens.dyndns.org/project_pages/electronics/pt100.html
And here is a Wikipedia article on RTD sensors that gives both good theoretical info as well as practical info.
So in conclusion you first have to wire up the RTD sensor into a proper measurement circuit before you worry too much about the software sketch details that will read the results of that external circuitry. We used a lot of RTD sensors in the refinery I worked at before retirement and we considered them the best overall temp sensor over the prior popular thermocouple types.
Lefty