Refining Recommendations

Speed on the first two, in my opinion. The main reason to run Neph's standard build something like SB Bonfire (or Lukas) is the fact that she's more hybrid as opposed to strictly enemy phase. +Spd helps her player phase utility, especially if she starts the turn with the Wrath 3 effect, so she can essentially start out defensive then transition after her initial encounter.

Olivia probably wants the speed too since she does miss a bit of speed if running +atk (I run that as well). And her def should be good enough for when she needs it, but she's not fully defensive at any rate.

I run the same olivia, and I refined her for speed. She kills things even when I DON'T want her to(training tower for example)

Effie's best choice is Berkut's Lance refined for Res. Makes her a wicked mix tank, especially with DC.