What are the six paths? In Naruto manga chapter 510, Madara mentioned the following two things:

1> 

2> 

So, what are the 6 paths that Madara was referring to, and how is Nagato the third one of them?
The six paths are related to the Sage of the Six paths and Pain. Recall that Pain had clones that each specialized in something? Well each one had contained one path from the six


Deva Path - Attraction and repulsion
Asura Path - Attack and defense
Human Path - Ability to read minds
Animal Path - Ability to summon
Preta Path - Absorbing Chakra
Naraka Path - Summon the Hell King


There is also the 7th path that Pain wasn't able to master, which was the Outer Path, the ability to control life and death. Pain used the Outer Path to revive the members of Konoha, however it costed him his life.

So when Tobi was talking about the 6 paths, he was talking about all the people that had control over the 6 paths. First would be the Sage of the Six paths, then it would be Madara, and lastly it would be Nagato.

On a side note, seeing as how he is not called the Sage of the Seven Paths, I think it is safe to assume he could not  control life and death. Meaning he would also probably have to sacrifice his life in order to use the Outer Path.