What is Fabers 3rd factor required for happiness

I think along the lines of intellectual freedom.

The ability to act on what you learned from the first two requirements.

Faber starts talking to him about what is missing in their lives -- Montag thinks it's just books, Faber says it is three things.

The third of the three things he mentions is something along the lines of freedom, the right to do things. He says that they need good information and the leisure to think about it (those are the first two) but then they also need the right to act on what they come up with when they think.

And number three: the right to carry out actions based on what we learn from the inter-action of the first two.

This would be something like freedom of the press, or speech, or action -- sort of all put together. They have to be able to have thoughts and then be allowed to act on those thoughts.