Karnaugh maps are usually drawn in Grey Code (with one bit changing at a time, so 000, 001, 011, 010 rather than with decimal sequence*) and if you do that, you get:
This corresponds to minterms
C'A'B + C'AB' + CA'B' + CAB = C' (A'B + AB') + C(A'B' + AB)
The XOR function is A ^B=A'B + AB' and A XNOR B is A'B' + AB
So f= C'(A XOR B) + C (A XNOR B) = C' (A XOR B) + C(A XOR B)' = C XOR (A XOR B) = A XOR B XOR C
- Note: You can only group together cells and reduce using a K-Map when it is in Grey Code sequence, because grouping two elements together (and discarding the variable that changes) correspons to
BC(A + A') = BC which only makes sense when one bit is changing.