3 You will keep in perfect peace those whose minds are steadfast, because they trust in you.

Read Isaiah 26:3 Using Other Translations

Thou wilt keep him in perfect peace, whose mind is stayed on thee: because he trusteth in thee.
You keep him in perfect peace whose mind is stayed on you, because he trusts in you.
You will keep in perfect peace all who trust in you, all whose thoughts are fixed on you!