This may be common knowledge, and correct me if this is wrong, but:
Unless you can hit confirm and be sure to launch with the second hit of scythe spin, allowing the third hit to land has been more advantageous for me for a couple of reasons. One being that the third hit will allow you to see if its going to be block and backdash for some extra space.
The second reason, and I think more important reason, is that there's a small gap between the second and third hit of the scythe spin. I took this in to practice and tested against the faster special move reversals (Lantern's Might) fast reversals with weird hitboxes (Batman's Slide), and even some more defensive reversals (Willpower Wall). I found that the gap will allow the opponent's move to start up, but will hit before their active hitboxes start. So, even if your opponent block your string and the beginning of the scythe spin, the third hit will still connect, allowing you to hit confirm that and bounce them for a juggle. Keep in mind that this works better while moving forward during the scythe spin.
Now, your opponent can make a read on this and use MB B3, or armor to get out of this, but this will open up headgames where you can back away during the scythe spin after the first hit if they're blocking and keep in optimal range.
