Because the total damage bonus from all strikes scales quadratically with the maximum, it wouldn't take much difference to make the Trisword evenly good across all levels. Maybe 5 at level 1, and increasing by 1 every two or three levels.
That might be interesting.
The Trisword should remain quite powerful when paired with a strength potion. This allows you a pretty powerful spike, with a damage bonus that tapers off as you go. However, after a few levels the +5 from drinking a potion isn't worth blowing a potion. This will make it impractical to leverage until the boss battle, at which point it can be sustained at its +7 power (which is pretty nice).
My first impression is that the new trisword is strong early and strong late, but for most of the game it's an inferior fine sword. I think that's fine for the item in and of itself, but I do have concerns for how this will impact class balance. Rogue and Monk can afford the nerf, but Crusader and Priest may have been overly-affected. This also is a pretty huge loss to the Halfling; it's probably going to knock my halfling usage down to dwarf levels.