1)
What should he tell Bnei Yehudah?
1.
Rashi: They should be here at the end of three days.
2)
Why did he say "v'Atah Fo Amod"?
1.
Rashi: Be here in front of me at the end of three days. (David wanted to chase after Sheva ben Bichri.) Radak - he wanted to appoint Amasa to be the head general.