Using potentiometer to measure distance in linear actuator
No, you don't measure resistance, you measure voltage. Apply known voltages (like 0V and 5V) to the two "reference" lines, and measure the "signal" voltage. There's an ADC in the Arduino to do that.

Internally ... a rotary pot is one way to do it. A more accurate way would be a linear pot running the full length of the actuator. How yours works I can't say.

If you needed an accurate position you'd use digital scales like a DRO or calipers instead.