Unfortunate Brave Lucinas

Yikes these are all unusable.. I suppose the +Spd -Atk is the best bet as she needs all the offensive stats she can get. If you aren’t going to use all 3 of them for Aether fodder (which is a much better investment IMO) then merge them into a +Spd one

I'm going to assume you meant to say you have a +Atk -Spd and a +Spd -Atk and a +Def - Atk.

I would merge her just because she's useful as a buffing cheerleader for melee units in the arena. In that capacity I would merge into the +Def -Atk, that way she survives initiations and ideally doesn't finish off the unit she's going after. But for everything else I would go with +Spd since her speed is excellent.

I'm pretty sure, there is no Brave Lucina. I spend so many Orbs, getting her, that I came to the conclusion, that she just is a myth (as is Faye and Regular Lyn).

I've pulled two Brave Lucinas over the course of about a year. Both were +Res/-HP. Used one, and the second one gave Lukas Aether. If you have someone who could use Aether, I'd do that.