Normally, the first terminal is ground, the second is variable resistance, and the third is full resistance. So normally, only pins 1 and 2 are used. Not all 3. And in answer to your first question, No. Get a bigger pot. Adding a resistor in series will just slide your impedance. In other words, you add a 100 Ohm resistor in, you get a pot that goes from 100 to 190 Ohms.