The clear objective of Moshe’s mission was that Bnei Yisrael leave Egypt unconditionally. Why, then, did Moshe misrepresent his real purpose by merely asking for a three day respite from their servitude so that they could serve Hashem? The Ra’Mah asserts that the strategy was deliberate. If Moshe were to ask Pharaoh to fully release the Jews from slavery, his request would have absolutely been denied. The mission, however, was a pretense to punish Pharaoh for not listening to Hashem.
If Pharaoh were to have been confronted with a demand that was perhaps “excessive,” like releasing an entire nation of slaves without warning, his refusal to comply might not have been considered unreasonable. Non-compliance to such an unreasonable demand might not have been grounds for punishment. Consequently, Moshe came to Pharaoh with a legitimate request, in order to avail the people an opportunity to celebrate a three day holiday. Pharaoh’s refusal to accede to such a simple demand demonstrated his heartlessness. He was now truly deserving of a more severe punishment.
Horav Chaim Freidlandler z.l., suggests that Hashem’s goal was that we should have no sense of gratitude whatsoever to Pharaoh. Our exodus from Egypt should manifest total subservience to Hashem. If Pharaoh had relented or had we stayed in Egypt longer, Pharaoh might have — of his own free will and “altruistic” nature — made some form of “peace agreement” with us. Consequently, we might have experienced a tinge of appreciation to him. By asking Pharaoh for a three day leave, Pharaoh was given the opportunity to think that he was permitting the Jews to leave Egypt. Three days was a demand to which Pharaoh could relate. When Bnei Yisrael stayed away longer than the originally allotted time, Pharaoh chased after them in order to bring them back. He was unsuccessful. By merely asking for three days, Bnei Yisrael precluded any possibility that they might feel a sense of gratitude towards Pharaoh.