To first clarify, even though lying is usually frowned upon, I'm pretty sure that, at least ethically speaking, there's no reason to frown upon lying to Pharaoh in this situation if it was necessary to save the Jews. The question being dealt with here is why was this deception necessary - couldn't God have saved them without the lie?
Thanks to this shiur, I've heard five separate answers to this question:
The Ramban addresses this question, and writes (comments to Shemos 3:18) that this message wasn't necessary for Pharaoh, but for the Jews. The Jewish people would have been unwilling to follow Moshe to the land of Canaan where so many strong and fearsome nations resided, until they saw God's mighty hand expressed in the incredible miracles that He performed. According to the Ramban, only after witnessing these miracles did Moshe tell the Jews the true intentions of leaving Egypt. Needless to say, this explanation is considerably difficult to read into the pesukim.
The Ibn Ezra and Ran in his Drashos write that telling Pharaoh that they would return after 3 days would (1) help convince the Egyptians to let them 'borrow' their stuff, and (2) cause Pharaoh to chase after the Jews when they didn't return, and thereby be drowned in the sea.
The Abarbanel disagrees with the Ran, since God hardened everyone's hearts anyway. He writes that this request was made of Pharaoh for PR reasons: by refusing such a reasonable request as a short vacation, everyone who hears the story will learn what an obstinate king Pharaoh was. R. Yaakov Tzvi Mecklenburg in HaKesav VaHakkabalah seems to agree, and he adds that this explains why Moshe stopped referring to the 'three days' request once the plagues got underway.
R. Yaakov Kamenetsy, in Emes L'Yaakov, suggests that the original plan was actually to only leave for three days. Even though eventually the Jews would be leaving Egypt in accordance with the bris bein habesarim, this would have only occurred after being in Egypt for 400 years, and a 'vacation' was needed of service to God to make sure that the Jews wouldn't sink to the depths of tumah during their slavery in Egypt, which would have continued. However, when Pharaoh refused and God had to begin bringing on the plagues, the plans changed.
The Rav Peninim Chumash includes many interesting 'peirushim' to various parshiyos, and his explanation given here is that God's instruction to deceive Pharaoh has an important educational value, to teach the Jewish people that in future wars they should do whatever is necessary to win, even if it may be deceitful or underhanded. Thus, even though deception wasn't necessary in this case, it will be in the future when we shouldn't be relying on miracles.