Which of the formulas is correct to find the equivalent of 3 inductors in a series circuit?

The formula to find the equivalent inductance of inductors connected in series is based on the fundamental principle that inductance in series adds directly. Therefore, when determining the total inductance (LT) for multiple inductors, you simply sum their individual inductances.

For three inductors connected in series, you calculate the total inductance by adding the inductance values of each inductor together: LT = L1 + L2 + L3. This approach reflects the accumulated effect of each inductor as they work together in the circuit.

The reason the other options do not apply is that multiplying or dividing inductance values does not correctly represent the physics of inductors in series. Each inductor contributes to the total magnetic field produced in the circuit, necessitating their values to be combined through addition to accurately determine the overall inductance.