Where did Cain's wife come from if there were only 3 people on the earth after Abel's death?

It seems that this is one of those subjects that have perplexed individuals for ages; Christians because there is no one definitive verse that clearly states who she was and agnostics who view this supposed omission as an alibi to “wrest the scriptures to their own destruction” (2 Peter 3:16As also in all his epistles, speaking in them of these things; in which are some things hard to be understood, which they that are unlearned and unstable wrest, as they do also the other scriptures, unto their own destruction.
See All...). However, if we compare scripture with scripture we can identify Cain’s wife with some simple biblical deduction and a little sound reasoning.

Some have suggested that there could have been other humans not begotten from Adam and Eve’s union around at this time but this simply cannot be true for Genesis 3:20And Adam called his wife's name Eve; because she was the mother of all living.
See All... states that Eve was the, “mother of all living.” According to the Biblical record, we also know that they were the only created human beings (Genesis 2:7And the LORD God formed man of the dust of the ground, and breathed into his nostrils the breath of life; and man became a living soul.
See All..., 22). Therefore, any humans that existed at that time had to be born from their union. This then leaves us with only one plausible explanation as to the identity of Cain’s bride; it must have been his sister.

The Bible doesn’t mention them having any daughters until Genesis 5:4And the days of Adam after he had begotten Seth were eight hundred years: and he begat sons and daughters:
See All... but obviously Eve was bearing them at or near the same time as Cain and Abel for in Genesis 4:17And Cain knew his wife; and she conceived, and bare Enoch: and he builded a city, and called the name of the city, after the name of his son, Enoch.
See All... Cain is said to already have a wife. In addition, common sense dictates that these daughters had to be born near or around the same time as the sons or else there would have been no way for the human species to procreate. With the Lord’s command for Adam and Eve, “to be fruitful and multiply,” (Genesis 1:28And God blessed them, and God said unto them, Be fruitful, and multiply, and replenish the earth, and subdue it: and have dominion over the fish of the sea, and over the fowl of the air, and over every living thing that moveth upon the earth.
See All...) and with their long life expectancy it was possible (if not probable) that Eve was able to bear more children (quintuplets, sextuplets, etc.) and at a faster rate then is possible for women presently. This would make the most sense in light of Genesis 3:16Unto the woman he said, I will greatly multiply thy sorrow and thy conception; in sorrow thou shalt bring forth children; and thy desire shall be to thy husband, and he shall rule over thee.
See All... where the Lord told Eve, “I will greatly multiply…thy conception.” Note that the first time Cain is said to know his wife that there is already enough of a population existing for him to establish and name a city after his firstborn son Enoch (Genesis 4:17And Cain knew his wife; and she conceived, and bare Enoch: and he builded a city, and called the name of the city, after the name of his son, Enoch.
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As to the problem with Cain marrying his sister this can be answered once again by searching the scriptures instead of relying upon our preconceived prejudices. At this time in the Bible it was permissible for a man to marry a sister or first cousin for in Genesis 20:12And yet indeed she is my sister; she is the daughter of my father, but not the daughter of my mother; and she became my wife.
See All... Abraham is said to have married his sister Sarah. It was not until the Law was given to Moses (see Leviticus 18:6-17 [6] None of you shall approach to any that is near of kin to him, to uncover their nakedness: I am the LORD. [7] The nakedness of thy father, or the nakedness of thy mother, shalt thou not uncover: she is thy mother; thou shalt not uncover her nakedness. [8] The nakedness of thy father's wife shalt thou not uncover: it is thy father's nakedness. [9] The nakedness of thy sister, the daughter of thy father, or daughter of thy mother, whether she be born at home, or born abroad, even their nakedness thou shalt not uncover. [10] The nakedness of thy son's daughter, or of thy daughter's daughter, even their nakedness thou shalt not uncover: for theirs is thine own nakedness. [11] The nakedness of thy father's wife's daughter, begotten of thy father, she is thy sister, thou shalt not uncover her nakedness. [12] Thou shalt not uncover the nakedness of thy father's sister: she is thy father's near kinswoman. [13] Thou shalt not uncover the nakedness of thy mother's sister: for she is thy mother's near kinswoman. [14] Thou shalt not uncover the nakedness of thy father's brother, thou shalt not approach to his wife: she is thine aunt. [15] Thou shalt not uncover the nakedness of thy daughter in law: she is thy son's wife; thou shalt not uncover her nakedness. [16] Thou shalt not uncover the nakedness of thy brother's wife: it is thy brother's nakedness. [17] Thou shalt not uncover the nakedness of a woman and her daughter, neither shalt thou take her son's daughter, or her daughter's daughter, to uncover her nakedness; for they are her near kinswomen: it is wickedness.
See All..., 20:17
See All...) that the marrying of blood relatives was forbidden. An interesting side note to this study is that a New Testament wife (1 Corinthians 9:15But I have used none of these things: neither have I written these things, that it should be so done unto me: for it were better for me to die, than that any man should make my glorying void.
See All...) and the bride of Jesus Christ (Song of Solomon 2:2As the lily among thorns, so is my love among the daughters.
See All..., 4:9
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