Inner Ability Resets

Does this means once it get to legendary, all 3 lines are at legendary, or still only top line is legendary worthy

Right now: First line is legendary, other two are rare/epic/unique (although you can't see what rank they are, and both lines aren't the same rank). When you circulate, the rank of the second/third lines doesn't go down so eventually they hit legendary rank and can get legendary bonuses, but you won't know when until you get something exclusive to legendary.
Later on: All lines are the same rank.

Yeah, I'm wondering if Weapon Speed can come out from either of the second or third line.

They can. They won't.