What to do with 3 Bikes?

I would keep +Atk -HP. As for Steady Breath fodder, I would use it on Fjorm or BK. Aether can wait another day unless you want to give that unit both Steady Breath (4* Silas comes in handy for stance fodder) and Aether (helps if the unit already learned Luna or inherited it from a 4* Catria or Frederick).