In one of the scanlated versions of Naruto manga chapter 510, Madara mentioned the following two things:
"Legend has it that Uchiha Madara was bested by Hashirama Senju... but is the legend true?"
"The true winner is he who chose on the future... and the real fight is about to begin."
"I fought that battle to gain access to his abilities."
"I am Uchiha Madara! The man who obtained Senju Hashirama's powers!"
"Two of the Six Paths are now one being!"
"You are the third of the Sixth Paths..."
"Look at you, you overexerted your powers so much that the Uzumaki clan's trademark red hair turned snow white."
So, what are the 6 paths that Madara was referring to, and how is Nagato the third one of them?