The data documented above are widely distributed, consistent, and sufficiently attested to allow meaningful generalisations. The clearest syntactic parallel to our verse is 2 Samuel 7:13, which is a promise to David of an offspring (zera‘, v. 12): hû’ yibenh-bayit lišěmî (‘he will build a house for my name’). From these data it becomes clear that, on the syntactical level, the singular pronoun hû’ in Genesis 3:15 is quite consistent with the pattern where a single individual is in view. In fact, since the subject pronouns are not normally necessary for the meaning, we might wonder if the singular hû’ in Genesis 3:15 is used precisely in order to make it plain that an individual is being promised, who will win a victory over the snake at cost to himself. The evidence of the Greek translators makes it beyond question that the translator of Genesis 3:15 meant to convey that an individual was promised; this study indicates that his interpretation is consistent with Hebrew syntax elsewhere in the Bible.
It should be admitted, however, that at this point we have used the data at the syntactical level, which is a fairly low level of integration. The move up to the exegetical level, to answer the question whether this is what one finds in Genesis 3:15, will depend on one’s view of other factors, such as how one’s overall perception of the context should interact with the particulars; whether the interpretation should be based on the final form of the text or on reconstructed sources; what range of ideological messages one is willing to ascribe to the work; and so on. To treat adequately any of these questions is outside the scope of this modest syntactical note.
If, however, one grants for the sake of discussion that it is a valid approach to make a 'bottom-up' reading of the final form of this text, which has a relationship to the larger picture of Genesis, then it would be fair to read this as God's threat to the snake, of an individual who will engage the snake in combat and win.
Please see Jack Collins, "A SYNTACTICAL NOTE (GENESIS 3:15): IS THE WOMAN’S SEED SINGULAR OR PLURAL?," Tyndale Bulletin 48.1 (1997) 139-148.