Some general confusion on if this is a -3 or a -2 penalty.
The player is a Level 1 Titan Fighter using the Giant-Blooded Campaign trait and is wielding a Large Quarterstaff.
I could see it either way. I think the issue is whether or not the Giant-Blooded trait interacts with the penalty imposed by the titan fighter's Giant Weapon Wielder ability.
My inclination is that the trait does apply. This is kind of powerful for a trait, but campaign traits do tend to be more powerful, so it's probably fine.
So, you would end up with a -2.