What's the minimum number of NOR gate needed to build a 3-input XOR gate using only NOR gate?

Information I know:

https://en.wikipedia.org/wiki/NOR_logic

A XOR B = [ ( A NOR A ) NOR ( B NOR B ) ] NOR ( A NOR B )

A XOR B XOR C = ( A XOR B ) XOR C

Using the above formulas, I can implement 3-input XOR with 20 NOR gates.

What I don't know:

What's the minimum number of NOR gate? Is there a simpler way to implement 3-input XOR gate?