When measuring ESR on a cap in series the result is just whatever the ESR of that specific cap is. If that cap happened to be in parallel does it then become the sum of all the ESR’s of the caps in parallel or does it become the average of the ESR’s of the caps in parallel?
Firstly, “in-circuit” could mean just about anything, and there may be lots of components influencing the measurement. . Resistors in parallel give a resistance that is lower than for any of the individual resistors, just google parallel resistors. Finally, ESR is a parameter it’s particularly difficult to measure in the circuit, but it may still be feasible, depending on surrounding components. If there is a resistor in series with the capacitor, you may get more accurate results by applying a signal to the live circuit, measure voltage drops and calculate ESR.
The ESR value of a capacitor say an electrolytic varies from value to value, make and type. As capacitors age the ESR figure tends to increase from the value when new. To gain a meaningful result one must first know what the specified ESR values is, at the test frequency stated in the manufacturers data.