Contradiction with Jeremiah 9:23
This verse advises not to glory in wisdom, might, or riches, while 3 John 1:3 commends walking in truth, potentially implying a form of glorying.
Jeremiah 9:23: Thus saith the LORD, Let not the wise [man] glory in his wisdom, neither let the mighty [man] glory in his might, let not the rich [man] glory in his riches:
Contradiction with Proverbs 3:5
Encourages trusting in the Lord with all your heart and not relying on your own understanding, which contrasts with placing emphasis on personal understanding of truth in 3 John 1:3.
Proverbs 3:5: Trust in the LORD with all thine heart; and lean not unto thine own understanding.
Contradiction with Galatians 1:10
Warns against seeking the approval of men, which could be seen as contrasting with commending the testimony of others as in 3 John 1:3.
Galatians 1:10: For do I now persuade men, or God? or do I seek to please men? for if I yet pleased men, I should not be the servant of Christ.
Contradiction with 1 Corinthians 10:12
Advises caution against being overconfident in one's standing, whereas 3 John 1:3 praises steadfastness in truth.
1 Corinthians 10:12: Wherefore let him that thinketh he standeth take heed lest he fall.