The flash-forward storyline, up til now, only featured two characters: an adult Tess and an older Randall, who are hesitantly going to visit an as-yet-unnamed "her." But in the season three premiere, Toby gets drawn into this storyline too, when Randall calls him and asks if he's coming to visit "her" too.
"She wants you to be there," Randall tells a hesitant Toby over the phone. This seems to suggest that Toby's arc this season is somehow tied to whoever "she" is — and that they are (or were) close enough that she would want to see him. Is "she" Kate, perhaps, or Rebecca? Or is it possible that the "she" Randall and Tess are visiting is separate from the "she" Randall is talking about to Toby? Either way, the "her" mystery is definitely connected to Toby's storyline now.