I mean think about it. Ign confirmed he is not at the start. Also look at the first trailer. You see Micheal and Franklin but no Trevor. Was he just added on to add more fun but not really a real story because we havent heard much about what he is trying to pursue..
I remember reading that he was an important part of the story, but not a playable character. In the end, they made him playable. So I guess you can call him an add-on, but he wasn't just put there for the hell of it.
Whatever you read was bullsh*t because it isnt true. The Benz already confirmed that they had in mind the 3 protagonists since San Andreas but the hardware was still primitive .