Exodus 6:3
3 And I appeared unto Abraham, unto Isaac, and unto Jacob, by the name of God Almighty, but by my name Jehovah was I not known to them.
Genesis 22:14
14 And Abraham called the name of that place Jehovahjireh: as it is said to this day, In the mount of the Lord it shall be seen.
In Exodus, God tells Moses that He was not known to Abraham, Isaac, and Jacob by the name Jehovah. This is an English transliteration of the Hebrew Tetragrammaton, YHWH, which is usually rendered "the LORD" in the Old Testament.
But, in Genesis 22:14 above, we have Abraham use the name Jehovah twice. The name LORD/Jehovah is also shown to have been known in Genesis 4:1, 4:26, 5:29, 9:26, 12:8, 13:4, 14:22, 15:2, 15:7-8, 16:2, 16:5, 16:13, 18:14, 19:13-14, 21:33, 22:16, 24:3, 24:7, 24:27, 24:31, 24:35, 24:40, 24:42, 24:44, 24:48, 24:50-52, 24:56, 26:22, 26:25, 26:28-29, 27:7, 27:20, 27:27, 28:13, 28:16, 28:21, 29:32-35, 30:24, 30:27, 30:30, 31:49, 32:9, and Genesis 49:18.
Clearly, the name Jehovah was known and used before Exodus 6:3. So, what is Exodus 6:3 referring to?
"Know", like thousands of other words, has more than one meaning. In context, "know" is referring to something more like recognition after a demonstration of power and ability.
To illustrate, Pharaoh, in the previous chapter, clearly knows the name Jehovah, yet says he doesn't "know" Him:
Exodus 5:2
2 And Pharaoh said, Who is the Lord, that I should obey his voice to let Israel go? I know not the Lord, neither will I let Israel go.
Similarly, in the immediate context of the passage in question, we see:
Exodus 6:6-7
6 Wherefore say unto the children of Israel, I am the Lord, and I will bring you out from under the burdens of the Egyptians, and I will rid you out of their bondage, and I will redeem you with a stretched out arm, and with great judgments:
7 And I will take you to me for a people, and I will be to you a God: and ye shall know that I am the Lord your God, which bringeth you out from under the burdens of the Egyptians.
Using the word "know" to signify a more complete understanding is also seen in the following passages in Exodus:
Exodus 7:5, 17
5 And the Egyptians shall know that I am the Lord, when I stretch forth mine hand upon Egypt, and bring out the children of Israel from among them.
17 Thus saith the Lord, In this thou shalt know that I am the Lord: behold, I will smite with the rod that is in mine hand upon the waters which are in the river, and they shall be turned to blood.
Exodus 8:10
10 And he said, To morrow. And he said, Be it according to thy word: that thou mayest know that there is none like unto the Lord our God.
Exodus 8:22
22 And I will sever in that day the land of Goshen, in which my people dwell, that no swarms of flies shall be there; to the end thou mayest know that I am the Lord in the midst of the earth.
Exodus 9:14
14 For I will at this time send all my plagues upon thine heart, and upon thy servants, and upon thy people; that thou mayest know that there is none like me in all the earth.
Exodus 10:2
2 And that thou mayest tell in the ears of thy son, and of thy son's son, what things I have wrought in Egypt, and my signs which I have done among them; that ye may know how that I am the Lord.
Exodus 14:4, 18
4 And I will harden Pharaoh's heart, that he shall follow after them; and I will be honoured upon Pharaoh, and upon all his host; that the Egyptians may know that I am the Lord. And they did so.
18 And the Egyptians shall know that I am the Lord, when I have gotten me honour upon Pharaoh, upon his chariots, and upon his horsemen.
Exodus 16:12
12 I have heard the murmurings of the children of Israel: speak unto them, saying, At even ye shall eat flesh, and in the morning ye shall be filled with bread; and ye shall know that I am the Lord your God.
Exodus 29:46
46 And they shall know that I am the Lord their God, that brought them forth out of the land of Egypt, that I may dwell among them: I am the Lord their God.
As can be seen from the above, this theme is referenced heavily in Exodus - God giving a demonstration of His power so as to sanctify himself to Israel. In their minds, He was creating an impression in ways that He had never done so before, for anyone, by the steady stream of miracles happening throughout the entire Exodus saga.
For an example of the word "know" being used to represent deeper understanding in the New Testament, we can see Jesus Christ tell the Jews, who certainly knew the name Jehovah, that they didn't know Him:
John 8:19
19 Then said they unto him, Where is thy Father? Jesus answered, Ye neither know me, nor my Father: if ye had known me, ye should have known my Father also.
John 14:7
7 If ye had known me, ye should have known my Father also: and from henceforth ye know him, and have seen him.
So, was God known to these people? In some ways, yes. In other ways, no. That's not a contradiction, because "knowledge" can be on a spectrum or a gradient.
While the patriarchs knew the name Jehovah/the Lord, they didn't have the same impression, recognition, or understanding of it that God made known to the generation that left Egypt in the Exodus.