There's Conflicts in the Definitions of XNOR?
XNOR is a negation of the Exclusuve OR on any number of inputs. Thus if you have n inputs, you perform

A0 + A1 + ... + An

discarding any carry bits from bit 0 (so that result remains to be 0 or 1), and then negate it.

Sounds like you are looking at the two-input implementation (e.g. from Wikipedia), and it is one of the possible ways to implement XNOR with two inputs. Three input equivalent (and optimized) circuit will look differently.