What is the XNOR of 3 or more inputs?
There is no standard definition for the XNOR of more than two inputs (indeed, nor is there a standard definition for less than two inputs). Since XNOR is associative, one possible definition is
$$
A_1 \text{ XNOR } A_2 \text{ XNOR } \cdots \text{ XNOR } A_n = A_1 \text{ XNOR } (A_2 \text{ XNOR } (A_3 \cdots )\cdots).
$$
Another reasonable definition defines the XNOR of $A_1,\ldots,A_n$ as the negation of the XOR of $A_1,\ldots,A_n$. Both definitions agree with the usual definition of XNOR when $n = 2$, or indeed when $n$ is even, but give complementary results when $n$ is odd.

The XOR of a given number of inputs is $1$ if the number of $1$s in the input is odd. Given that you can work out a similar formula for the XNOR of any number of inputs, under both definitions.