Who should get Life and Death 3 from Sothe?

Is he your only sothe? Well if you're not interested in him as a unit I'd probably go with spring kagero or fallen celica ( does she have an a slot xd?)

Many options here, it mostly depends on who you use and how comfortable you are with their lowered bulk.

Generic options
- Spring Kagero for full sweeper
- Valentines Lyn to make her a full Linde-like glass cannon
- Morgan for sweeper, her speed will be usable with team buffs
- P!Olivia to provide offense when not dancing, will want a boosting dagger like Poison or Kitty Paddle
- Takumi/Clarisse, though you already have Bridal Cordy.
- F!Celica, her weapon's effect will mitigate the loss in bulk
- Zelgius for a good speed boost at the cost of his bulk
- Guunthra, though this will reduce her ability to use non chilling seal ploys
- Fjorm could use the speed boost, and it will increase her damage taken, allowing her to get a larger damage boost out of Ice Mirror.