24 And the Lord did so; and there came a grievous swarm of flies into the house of Pharaoh, and into his servants’ houses, and into all the land of Egypt: the land was corrupted by reason of the swarm of flies.

25 And Pharaoh called for Moses and for Aaron, and said, Go ye, sacrifice to your God in the land.

26 And Moses said, It is not meet so to do; for we shall sacrifice the abomination of the Egyptians to the Lord our God: lo, shall we sacrifice the abomination of the Egyptians before their eyes, and will they not stone us?

27 We will go three days’ journey into the wilderness, and sacrifice to the Lord our God, as he shall command us.

Pharaoh held out during the plague of lice, but not against the flies. After the frogs he had told Moses “take away the frogs from me, and from my people; and I will let the people go, that they may do sacrifice unto the Lord.” This time he again approved of the Israelites making sacrifice, but he added the restriction that they remain “in the land,” rather than travelling away from Egypt.

The fact that Pharaoh was being more specific and asking the Israelites to compromise might mean that he was more sincere this time than the one before, but his restriction was not acceptable to the Lord. There seem to be three reasons why it was necessary that the Israelites journey three-days distance before making their sacrifices.

1. As Moses explained, the Israelites would be performing sacrifices that the Egyptians found abominable, likely because the animals were considered sacred to Pharaoh’s people. The Israelites would incur the wrath of the people, thus a separation was necessary to preserve their lives.
2. There is reason to believe that sacrifice and three-days’ distances had something to do with the law of the Hebrews. In the Dead Seas Scrolls there is a passage that states “You shall not slaughter clean cattle or sheep or goat in any of your towns, within a distance of three days’ journey from my sanctuary” (Dead Seas Scrolls translation, Page 254, bottom-left). This is saying that if an Israelite was within three days’ journey of the temple they needed to make their offerings there. But if they were more than three days’ journey away, they would be permitted to build an altar and sacrifice to the Lord elsewhere. It may well be that Jacob was three days’ journey from Isaac’s home when he made such an altar to the Lord in Genesis 28. Jacob also made a point of setting up his homestead three days’ journey from his father-in-law Laban, perhaps to have sufficient distance from Laban’s pagan gods for his offerings to the Lord to be pure. It therefore makes sense that the Lord would require Israel to be the same distance from the false gods of Egypt before making sacrifice.
3. We’re not actually told whether the Israelites did, in fact, perform rituals and sacrifice after leaving Egypt. My assumption is that they did not, as we are told that they marched forward day-and-night in great haste. But possibly they did, and thus this was a valid reason for them traveling at least three-days-distance into the wilderness. However, even if that was the case, it obviously was not the only reason for their departure. They were also going into the wilderness to run away from the Egyptians, and obviously this was better served by getting a head start.

Pharaoh would soon renege on his promise, and perhaps his reason was this three days’ distance requirement, but God was not here to compromise. He was not one to be dictated to. What He required must be met, and He would continue to afflict the Egyptians until Pharaoh agreed to all of His requirements.