Why did Moshe need to keep using the cover story of "Let us go out to the desert for three days to worship our God" - why not just say "Let us go free", or the more popular "Let my people go!"?
Pharoah knew what Moshe really wanted as he said in 10:11 that only the men should go - he knew they really just wanted and were planning to leave under the guise of religious ceremony, and Moshe probably knew he knew. So why the game of "let us go serve our God for three days"?
I can understand initially they asked for three days because maybe then asking to leave was too much and there was no way Pharoah would have ever approved. But now, after a number of plagues have already decimated Egypt? After so much hositility? Just say "Let my people go!"???