English Translations: (See what sort of love the Father has given to us: that we should be called God's children--and indeed we are! For this reason the world does not know us: because it did not know him. (NET Bible)
See what marvellous love the Father has bestowed upon us--that we should be called God's children: and that is what we are. For this reason the world does not recognize us--because it has not known Him. (Weymouth)
See what sort of love the Father has given us, that we should be called children of God! And that is what we are. That is why the world does not know us, because it has not come to know him. (NWT 2013)
See what love the Father has given us, that we should be called children of God's, and are such! This is why the world does not know us because it did not know him. (Byington)
Comments: καὶ ἐσμέν.
From the NET Bible: "tc The phrase καὶ ἐσμεν (kai esmen, 'and we are') is omitted in 049 69 Ï. There seems to be no theological reason to omit the words. This has all the earmarks of a classic case of homoioteleuton, for the preceding word (κληθῶμεν, klhqwmen, 'we should be called') ends in -μεν (-men)."
Regarding the words, ὁ κόσμος οὐ γινώσκει ἡμᾶς, ὅτι οὐκ ἔγνω αὐτόν, we read:
"sn The pronoun him is a clear reference to Jesus Christ (compare John 1:10)." This statement is also found in NET.
However, the pronominal "him" is not all that clear, like many such references in the First Johannine Epistle. W. Harris Hall III explains:
"The referent of αὐτόν (auton, 'him') in 3:1. Again the referent of the third person pronoun is a problem. It could refer either to God the Father or to Jesus Christ, but since the Father is clearly mentioned in 3:1a and God is mentioned in 3:2a, it seems preferable to understand αὐτόν in 3:1b as a reference to God the Father. However, it is important to remember that Johannine christology associates Jesus with God, and there may have been little difference here as far as the author was concerned."
While I don't accept his Trinitarian presuppositions, there is likely a measure of truth to this observation.
Henry Alford avers: "because it did not know Him (viz. God: the Father."
He additionally states:
But Whom did the world not know, and when? αὐτόν here, by the very requirements of the logic of the passage, must be the Father, who not being recognized, neither are His children: τὸν υἱοθετήσαντα, as Œc.; Aug(38), Benson, al., understand Christ: “ambulabat et ipse Dominus Jesus Christus, in carne erat Deus, latebat in infirmitate.” But this can only be, if we understand that the world rejected that revelation of the Father which was made by Christ His Son. And if we introduce this element, we disturb the strictness of the argument. It is the world’s ignorance of God, considered (and this is the force, if it is to be pressed, of the aor. ἔγνω) as one great act of non-recognition, disobedience, rebellion, hate (for all these are involved in St. John’s οὐ γνῶναι, as their opposites in his γινώσκειν), which makes them incapable of recognizing, loving, sympathizing with, those who are veritably children of God: cf. ch. 1 John 5:1).