Contradiction with Matthew 6:1
This verse advises against doing good deeds publicly to be seen by others, contrasting with the act of helping brethren as encouraged in 3 John 1:5.
Matthew 6:1: Take heed that ye do not your alms before men, to be seen of them: otherwise ye have no reward of your Father which is in heaven. [alms: or, righteousness] [of your: or, with your]
Contradiction with Luke 6:35
This verse emphasizes loving enemies and doing good to them, which contrasts with 3 John 1:5's focus on doing good to brethren.
Luke 6:35: But love ye your enemies, and do good, and lend, hoping for nothing again; and your reward shall be great, and ye shall be the children of the Highest: for he is kind unto the unthankful and [to] the evil.
Contradiction with Proverbs 27:10
Advises not to forsake your friend or your father's friend, which is different in focus from the hospitality and support towards brethren and strangers in 3 John 1:5.
Proverbs 27:10: Thine own friend, and thy father's friend, forsake not; neither go into thy brother's house in the day of thy calamity: [for] better [is] a neighbour [that is] near than a brother far off.