Why is it not a sin when God asked Moses to lie to Pharaoh? And how does this expand on our conception of what is sin?


  (Exodus 3:18) ...and you shall say to him, ‘The Lord God of the
  Hebrews has met with us; and now, please, let us go three days’
  journey into the wilderness, that we may sacrifice to the Lord our
  God.’
God didn't lie

God knew that Pharaoh would say "no" to this request, and since we know the outcome of the story we can think that God intended for Israel to stay put all along and never intended them to journey into the wilderness. But the simple fact of the matter is we cannot assume that. We can assume that God did intend for the Israelites to go into the wilderness to worship him and there's nothing in the text that contradicts that.

Then what about this portion?


  Exodus 3:17,19 ESV "...I promise that I will bring you up out of the affliction of Egypt to the land of the Canaanites, the Hittites, ... , a land flowing with milk and honey. ... But I know that the king of Egypt will not let you go”


Wouldn't He be deceptive by asking the elders to request leave from Pharaoh for reasons other than the one's he's stated here?

All God is doing is proving his omniscience. He is declaring that He knows what the final result of all this will be. He knows he will lead them out and he knows Pharaoh will resist, but this doesn't change the requests he has for the Israelites or the expectations he has for Pharaoh. One way to look at it is God is giving an opportunity to Pharaoh to say yes and save Egypt a whole lot of trouble (and death). God even gives Pharaoh 9 more opportunities to let them go. God is merciful.

It's best to interpret passages like this with the question "In what reasonable way could I interpret this to be true?"