variable resistor instrumentation amp
There is a standard 3 OP instrumentation amplifier circuit, that has a single gain determining resistor. You should be able to find the circuit easily, or look for the TI INA128 as a single chip version of this circuit.
Typically, an instrumentation amplifier is designed with fixed precision resistors (e. g. 1% or 0.1% accuracy) rather than variable resistors. With a LM324, the accuray would mainly suffer from offset voltages and offset drifts. Thus designig the instrumentation amplifier with 1% resistors provides a better accuracy than this OP could ever achieve.