Electronic – Confusion with Wheatstone bridge resistor terminology in datasheets

You really don't have much choice here. There are two options:

1. amplify
2. Sample using some technique such that the 6.6mV signal can be seen with enough precision.

Using a 3V reference, With 10 effective bits, you'll get about 3mV resolution, so your entire output range will be covered in 2 least significant bits. That doesn't sound like enough! With a 24-bit A/D that give you about 17 effective bits, you'll get about 23uV, and your range will be covered in about 260 least significant bits. That might be enough. Does being able to divide your output range into 260 chunks solve your problem?

Now you have to consider your noise levels to make sure your SNR is OK. You can use a smaller reference voltage, of course, but I haven't had much experience with reference voltages at tens of millivolts, which is where you'd likely need to be.

What is the sampling/measurement arrangement that drives your concern that 6.6mV isn't good enough?? I suggest you start at the other end. What resolution, in terms of a pressure change, do you need to reliably identify? Once you know that, you can design a measurement system that can meet that challenge.

PT100 can be used as 3-wire or 4-wire based on the RTD you have. There are different options available and which one you need to use depends on the precision required.

As far as Wheatstone bridge is concerned, the idea lies between balanced and unbalanced bridge. Check wiki link for more details on bridge.

I don't know how to connect the Pt100 wires in order to get my Wheatstone Bridge.

The variable resistance part R2 shown below is your PT100

Image source: Wikipedia

You should use R1, R3 and Rx and 1% 100 ohm resistors for good results. The voltage change is measured along the terminals D and B. When the bridge is balanced you get almost zero voltage between terminals B and D. And when there is variation in the temperature, the resistance value of PT100 changes which in turn makes the bridge unbalanced and you get few mV between terminals D and B.

The important part here is the excitation i.e. the voltage applied at terminal A of the Wheatstone Bridge, it must be constant current source to give good and precise results, you can use darlington pair or opamps to do constant current source design. Refer this thread from TI which talks about constant current source and measurement techniques.

For a good and robust design and a signal conditioning circuit must come after the Wheatstone Bridge and ADC which converts you analog temperature to digital useful data. See here application note from Microchip which talks about signal conditioning for RTDs.

And even there are integrated options available from different Silicon manufactures for temperature sensing using RTD or thermocouple like LMP90080 and other are available from Analog Devices.

And at last please don't build sensing circuits on breadboard as it will add lead inductance to circuit and will surely affect performance.

Hope this helps you.