A 3-input OR gate can be made with an LM3900 amplifier, as shown in the figure. The purpose of the 150k ohms resistor at the inverter input is to keep the circuit saturated at a low output voltage when all inputs are low (0 V). The current flowing through the 75 k ohm resistors will change the state circuit, bringing the output to a high level. The fan-out of this circuit is high since each input has a resistor of 75 k ohms. This fan-out is equal to 50. The circuit can be easily transformed into a NOR gate by changing the inputs.
