What does across mean?
Does it mean between Phases A and C?
OR
Does it mean in phase A and on phase C?
What does across mean?
Does it mean between Phases A and C?
OR
Does it mean in phase A and on phase C?
Here, "across" means "between". In other words you connect one end of the impedance to phase A and the other to phase C.
I don't know what you mean by "in phase A and on phase C", but whatever that means, it isn't the correct interpretation.