When did Yehoshua say this?
Rashi: It was on the third day (Nisan 9); the 30th day of mourning over Moshe was the first day.
What is the meaning of "Hiskadsahu"?
Rashi: Prepare [Radak
What will happen tomorrow?
Rashi: They will cross the Yarden.
What are the "Nifla'os"?
Rashi citing Targum Yonasan: It is Prishan. 1
Radak: The Yarden will split, and the water will pile up.
How did Yehoshua know about the miracles that will happen? Hashem did not tell him!
Radak: Hashem told him three days ago to prepare, for they will cross the Yarden. It was full! Yehoshua understood that it must be via miracles.
Malbim: Normally, Leviyim carried the Aron. Now Kohanim would carry it. Yehoshua understood that this is due to miracles that will occur. Kohanim carried it three times (refer to 3:3:2:2 and the note there), and each time, miracles occurred. And Hashem commanded Yisrael to sanctify themselves, so the miracle will be in their merit.