It is well known that a three‐level system functions as a mixer for radiation at its three resonant frequencies. Here the topic is treated in detail by analogy to a three terminal‐pair circuit element, one pair for each frequency. To one pair, fixed power is applied from a local oscillator, or ``pump.'' Attention is then centered on the other two pairs to determine their small‐signal response and transfer characteristics. A slight generalization of the impedance concept takes account of the frequency changes and allows the results to be expressed as self‐impedance at each terminal pair, and a transfer impedance between pairs. The damping effect of molecular surroundings is taken into account. The three‐level maser is a special case in which the terminal pair for the idle frequency is open, and the self‐impedance at the signal terminals has a negative resistive part.Quantization of the radiation field is included by expressing quantum uncertainties in the same form as classical noise, i.e., correlation functions and power spectra. The means for doing this comes from a formalism which connects noise theory to quantum theory through the fact that the Wigner density of linear systems, such as modes of the radiation field, varies with time exactly like a classical probability density. The rules for using this technique are stated as needed without proof. The photon concept is not used.The method is extended to include a system pumped by broad‐band noise.