The following table will help you understand the discussion of 3:15 below.
1) (Hebrew text) In 3:15 it will crush your head, God is telling the serpent that Eve’s offspring will bruise/crush its head. It tells of the struggle between humankind and evil in which we will be finally victorious over evil. Our Bibles, and our Mass texts, are translations of the Hebrew text.
2) (Greek LXX text) he will crush your head This verse was interpreted in a messianic sense, ie as predicting the Messiah, by many Latin fathers of the Church because the LXX translation (Greek OT ie the Septuagint) does not have ‘it will bruise your head’ but ‘he will bruise your head’. S. Mowinckel He That Cometh page 11 rules out the Hebrew ‘it will bruise your head’ as a messianic passage and Becker Messianic Expectation in the Old Testament page 35 says only the LXX text is messianic. In other words, the Hebrew text does not refer to Jesus while the Greek text does.
Paul in Rom 5:12-21 develops this christological reading of Gen 3 in conjunction with the doctrine of original sin. Rom 5:19 states ‘Just as by one man’s disobedience many were made sinners, so by one man’s obedience are many to be made upright’. 1 Cor 15:21-22 is similar. The first sin was committed in the Garden of Eden and this was rectified by Jesus, the New Adam, in another garden, Gethsemane, who also was tempted in the garden; ‘My Father, if it is possible, let this cup pass me by. Nevertheless, let it be as you, not I, would have it’ (Matt 26:39). The sin of disobedience in the first garden was rectified by the obedience of Jesus in the second garden. The Catechism of the Catholic Church (CCC) 412 states that what Jesus won for us was greater than what Adam lost for us. Reading Gen 3 from a christological perspective we see Jesus as the New Adam who atoned for the first Adam. The Exultet (Easter Proclamation) which is proclaimed just after the lighting of the candles on Holy Saturday night says:
What good would life have been to us,
3) (Latin text) she will crush your head As well as reading Gen 3 christologically there has been a tradition in the Church since the early centuries to read Gen 3:15 mariologically, regarding Mary as the New Eve just as Jesus is regarded as the New Adam. The Latin Vulgate translation is different again, ‘she will crush your head’ and in the context of the text’s messianic interpretation by Latin fathers ‘she’ is taken to refer to Mary. That is the reason why this excerpt of Genesis is chosen as our First Reading on the Feast of the Immaculate Conception. The struggle between Mary and the devil is therefore what Gen 3:15 is said to be alluding to, see CCC 411. However this verse hints at ultimate victory. Because that verse in Genesis is the first announcement in the Bible of our salvation by Jesus it is referred to as the Proto-evangelium, meaning the ‘first Gospel’, the first glimmer of salvation. The different translations have different meanings.