(18) They will listen to you; then you shall go with the elders of Israel to the king of Egypt and you shall say to him, ‘The LORD, the God of the Hebrews, manifested Himself to us. Now therefore, let us go a distance of three days into the wilderness to sacrifice to the LORD our God.’
(3) They answered, “The God of the Hebrews has manifested Himself to us. Let us go, we pray, a distance of three days into the wilderness to sacrifice to the LORD our God, lest He strike us with pestilence or sword.”
(26) the LORD said to Moses, “Go to Pharaoh and say to him, ‘Thus says the LORD: Let My people go that they may worship Me. (27) If you refuse to let them go, then I will plague your whole country with frogs.
There is no doubt that this request was a diversionary tactic, the intention was never to return after three days. After Pharaoh had enslaved them without legal president, there is no reason to wonder why G-d commanded to deal with them in a deceiving manner.
There is no doubt that if Moshe refused so adamantly to go as it was, he would have refused more steadfastly had G-d commanded him to command Pharaoh to send out the Jewish people permanently. Moshe would have completely refused if that has been the case, because he didn't have the courage to say such a thing to the king: "You have been bad to this nation, now let them go!".
Perhaps if he would say such a thing to Pharaoh, Pharaoh would have killed him. Perhaps he (Pharaoh) would have been even more cruel to to the Jewish people in a far worse manner than he did in reality by increasing the work and making the bricks.
The Ba'al Haakeida writes that this was a deep and Godly advise to everyone to show the the strength and stubbornness of Pharaoh, in that all that was asked was to loosen his hands on them for about 10 days - to go for 3 days, sacrifice (and then return). It is logical to assume that they were going to return, and even in such conditions he was unable to hear their arguments, how much more so would this be the case if the request was to set them free completely.
This is not correct, because the main reason of Pharaoh's refusal was his concern that they would not return, as the verse says (10:8) "Who will be going".... , we can't find Pharaoh guilty of this.
The students of Rabbi Yitschak Pardo have answered that if Pharaoh had immediately asked for a guarantee that they would return, he would have been in the right, however, he only asked for a guarantee after the plagues had struck. However, beforehand not only did he ask for a guarantee of their return, but made life harder for them, which shows us the evil in his heart.