XNOR (Chip)
The XNOR gate is a digital logic gate whose function is the logical complement of the XOR gate. A 1 (high) output results if none or two of the inputs to the gate are 1 (high). If one or three inputs are 1 (high), a 0 (low) output results.
A way to remember XNOR is "the number of 1 (high) inputs is even".
If no signal is connected to an input pin, the input is read as 0 (low). This is done by internal pull-down resistors that pull the signal to low in the absence of an input signal. Using this mechanism the XNOR gate can be used with 2 or 3 inputs in the exact same way.
Play with the 3 buttons to see the function of the XOR gate. You will see that the output is only 1 (high) if one or all three inputs are 1 (high), in any other case the output will be 0 (low).