Albert Barnes Commentary Exodus 3:18

"And they shall hearken to thy voice: and thou shalt come, thou and the elders of Israel, unto the king of Egypt, and ye shall say unto him, Jehovah, the God of the Hebrews, hath met with us: and now let us go, we pray thee, three days` journey into the wilderness, that we may sacrifice to Jehovah our God." — Exodus 3:18 (ASV)

Three days’ journey – that is, a journey which would occupy three days in going and returning. This was a demand quite in accordance with Egyptian customs. The refusal of Pharaoh and the subsequent proceedings were revealed to Moses at once; but it is important to observe that the first request which Pharaoh rejected could have been granted without any damage to Egypt, or any risk of the Israelites passing the strongly-fortified frontier.