Ecclesiastes 4:12

And if one prevail against him, two shall withstand him; and a threefold cord is not quickly broken.

Cross-reference

In 2 Samuel 23:16, three mighty men break through enemy lines for David — a concrete example of three acting together with strength, like the unbreakable cord.

In Ephesians 4:3, believers are urged to maintain unity in the bond of peace — a similar principle of cohesive strength.