I know, but you can't ignore Chapters 1 & 2 when attempting to understand Chapter 3. Those chapters are clearly about sins that Christians commit being forgiven.
>1st John 3:2 Beloved, now are we the sons of God, and it doth not yet appear what we shall be: but we know that, when he shall appear, we shall be like him; for we shall see him as he is. 3 And every man that hath this hope in him purifieth himself, even as he is pure. 4 Whosoever committeth sin transgresseth also the law: for sin is the transgression of the law. 5 And ye know that he was manifested to take away our sins; and in him is no sin. 6 Whosoever abideth in him sinneth not: whosoever sinneth hath not seen him, neither known him. 7 Little children, let no man deceive you: he that doeth righteousness is righteous, even as he is righteous. 8 He that committeth sin is of the devil; for the devil sinneth from the beginning. For this purpose the Son of God was manifested, that he might destroy the works of the devil. 9 Whosoever is born of God doth not commit sin; for his seed remaineth in him: and he cannot sin, because he is born of God. 10 In this the children of God are manifest, and the children of the devil: whosoever doeth not righteousness is not of God, neither he that loveth not his brother. 11 For this is the message that ye heard from the beginning, that we should love one another. 12 Not as Cain, who was of that wicked one, and slew his brother. And wherefore slew he him? Because his own works were evil, and his brother’s righteous.
Verse 3 tells you purify yourself by avoiding sin. This is something you want to strive for because sin is of the devil. That's the only way I can rationalize it with the earlier parts of the letter. Perhaps 'cannot sin' means 'are not allowed to sin'.