What does Psalm 17:3 mean?

It is natural, but not rational, to complain or blame God when trials add up. David did neither of these. He believed the trials he encountered were intended, by God, for a reason. That purpose, he felt, was to test the spiritual quality of his life. From that perspective, David confidently claims that God's testing turned up nothing amiss. David was aware of his own sin (Psalm 143:2). Still, he knew sin was not the typical pattern of his life (Deuteronomy 18:13; Proverbs 29:10). Among the commitments David had made to God, one was to guard his words. In this context, this means he would not complain nor blame the Lord for what befell him.

The Lord tests believers today, too, but how believers respond to the discipline is extremely important. Tested believers should regard the disciplining as the Father's way of correcting His children and improving their character. Hebrews 12:5–6 advises believers to "not be weary when reproved by him. For the Lord disciplines the one he loves, and chastises every son whom he receives." The same passage tells us why God the Father tests His children: "For the moment all discipline seems painful rather than pleasant, but later it yields the peaceful fruit of righteousness to those who have been trained by it" (Hebrews 12:11).