No, not - See the marginal rendering. Others explain it to mean, Pharaoh will not let the people go even when severely smitten.

I am sure that the king of Egypt will not let you go, no, not by a mighty hand - When the facts detailed in this history have been considered in connection with the assertion as it stands in our Bibles, the most palpable contradiction has appeared. That the king of Egypt did let them go, and that by a mighty hand, the book itself amply declares. We should therefore seek for another meaning of the original word. ולא velo, which generally means and not, has sometimes the meaning of if not, unless, except, etc.; and in Becke's Bible, 1549, it is thus translated: I am sure that the kyng of Egypt wyl not let you go, Except wyth a mighty hand. This import of the negative particle, which is noticed by Noldius, Heb. Part., p. 328, was perfectly understood by the Vulgate, where it is translated nisi, unless; and the Septuagint in their εαν μη, which is of the same import; and so also the Coptic. The meaning therefore is very plain: The king of Egypt, who now profits much by your servitude, will not let you go till he sees my hand stretched out, and he and his nation be smitten with ten plagues. Hence God immediately adds, Exodus 3:20 : I will stretch out my hand, and smite Egypt with all my wonders - and after that, he will let you go.

And I am sure that the king of Egypt will not let you go,.... Or "but" (c) "I am sure", &c. though so reasonable a request was made him, yet it would not be granted; this is observed to them, that they might not be discouraged when he should refuse to dismiss them, which the omniscient God knew beforehand, and acquaints them with it, that, when it came to pass, they might be induced to believe that the mission of Moses was of God, rather than the contrary:

no, not by a mighty hand; the mighty power of God displayed once and again, even in nine plagues inflicted on him, until the tenth and last came upon him; or "unless by a mighty hand" (d), even the almighty hand of God; prayers, entreaties, persuasions, and arguments, will signify nothing, unless the mighty power of God is exerted upon him.

(c) "ego autem", Junius & Tremellius, Piscator; "sed ego", V. L. (d) Sept. "nisi", V. L. Pagninus, Vatablus; so Noldius, p. 344. No. 1246.

And I am sure that the king of Egypt will not let you go, no, not by a mighty hand.

3:19 I am sure he will not let you go - God sends his messengers to those whose obstinacy he foresees, that it may appear he would have them turn and live.

no...: or, but by strong hand

3:16-22 Moses' success with the elders of Israel would be good. God, who, by his grace, inclines the heart, and opens the ear, could say beforehand, They shall hearken to thy voice; for he would make them willing in this day of power. As to Pharaoh, Moses is here told that petitions and persuasions, and humble complaints, would not prevail with him; nor a mighty hand stretched out in signs and wonders. But those will certainly be broken by the power of God's hand, who will not bow to the power of his word. Pharaoh's people should furnish Israel with riches at their departure. In Pharaoh's tyranny and Israel's oppression, we see the miserable, abject state of sinners. However galling the yoke, they drudge on till the Lord sends redemption. With the invitations of the gospel, God sends the teaching of his Spirit. Thus are men made willing to seek and to strive for deliverance. Satan loses his power to hold them, they come forth with all they have and are, and apply all to the glory of God and the service of his church.