edit: lol beaten. this is what happens when I take 41 minutes to write a reply
Isn't her 3 (double kick move) unsafe on block? Maybe I was reading the frame data wrong but I thought it was. The first part is insanely short ranged though, so I'm not sure where I'd apply this even if it was safe.
Multi-hit moves were bugged until the latest patch. The game now correctly displays it as 0 on block.
As for its uses:
- Used after cross-up jump-in attacks where even you can't guess the direction. Just hit 3 and the move will come out regardless of what side you land on. F2 after cross-up can lead to B2 if your reactions are terrible.
- Pretty good damage (7.85% in total).
- Low combo damage scaling. Only reduces follow-up combo damage by 9.75% compared to the approx 35% of 223, 28% of f222 (higher if the opponent is already in the air), approx 19% of 22. Not much of a benefit in short combos, but gets better as the combo extends.
- Unlike 223, there's no gaps so you don't need to worry about being parried in the middle of the combo. Annoyingly, there is a gap between the second hit and Singularity (but not Soul Crush) so it can be back-dashed out of or punished with super moves.
Not the best move in the world, but any time you were going to do 223 into a special, 3 into the same special will net you more damage.
223, db2 = 15% damage
3 (1 hit)~db2 = 14% damage and far better scaling than 223
3 (2 hits)~db2 = 17% damage and still far better scaling than 223
The later in a combo it appears the less of an effect the reduced damage scaling has. Use it early or don't bother. After ending a combo with f22u1 if I feel like aiming for more damage by burning meter, I used to follow up with:
ji2 (no idea which side this will hit), 22~db2 MB, b3, ji2, f22u1 for 35%.
However, I recently switched to:
ji2 (again, no idea) 3 (both hits)~db2 MB, b3 ji2, f22u1 for 41%.
6% more damage for the same number of hits and 1 less input

. There are probably more optimal just-frame combos, but I'm comfortable with this one and can do it reliably. The only time it ever goes wrong is if I input db2 too early and cancel the first hit of 3 instead, but all that does is drop the damage down to 40% which is still more than the previous one.
edit: oh lordy lordy, finally made ji3 connect after the b3 into f22u1. Now at 43% \o/