There are a few possibilities:
Moses starts off boldly but is intimidated by Pharaoh’s refusal in Exodus 5:2, so he asks more humbly in Exodus 5:3.
Moses starts wrongly. What he says in Exodus 5:1 was not commanded by God, in Exodus 5:3 he corrects that. Exodus 5:3 is in line with God’s command in Exodus 3:18.
Moses speaks boldly and clearly in Exodus 5:1. In Exodus 5:3 he explains those words.
The third possibility seems the most likely. Moses makes no concessions, neither is he punished by God for what he has said (not even afterwards, Exodus 6:1). In Exodus 5:1 he gives a clear command from God. In Exodus 5:3 he tries to persuade Pharaoh with arguments. Though the us
in lest he fall upon us with pestilence or with the sword
is ambiguous, this us
can also refer to the Pharaoh and the Egyptians. That also happens when the plagues follow.
3 Then they said, “The God of the Hebrews has met with us. Please let us go a three days’ journey into the wilderness that we may sacrifice to the LORD our God, lest he fall upon us with pestilence or with the sword.”