- clearly in the book - Sony heads up as the third capo, this is spelled out in the text.

I would think Fredo is pre-assassination attempt a trusted aide-de-camp

A recognized, not that usefull for big tasks, but able to handle small ones, type of guy. Directly helping his father probably pleases him, and he is totally trusted at the time. (he is never a suspect in the attempt on the Don for instance - in the movie his love of Vito is more than played out)

His role post move to Las Vegas? - probably to act as the caretaker of a bunch of Moe Green's old built up operations that will eventually fade away. From the dialog - he clearly runs some local operations and has people.

He probably thinks of himself as an old-school Capo - but Michael is playing at much higher stakes - like CUBA and US politics