What does Psalm 9:3 mean?

The phrasing of this verse speaks of the present, but the events in question are in the past. This is a poetic choice, used in modern language even today. Depicting some dramatic event in history, a writer might switch to a present-tense narrative for the sake of drama. Context clarifies that David is celebrating past success, as the phrase "you have" occurs five times between verses 4 and 5.

God completely confounded David's enemies. The imagery here is someone retreating in such a hurry that they trip and fall over themselves, and so are even more easily defeated. A similar depiction of absolute terror is presented in Psalm 21:12.

Throughout history enemies have tried to destroy the people of Israel, but the Lord has turned them back just as He turned David's enemies back. Egypt tried to destroy God's people, but God intervened and destroyed the Egyptian army at the Red Sea (Exodus 14:27–29). Both before David's life (Judges 2:11–15), and after (Lamentations 1:1–9), God allowed Israel to suffer defeat and conquest because of their sin. However, they never faced annihilation.

During the tribulation period, Satan will try to destroy Israel, but will fail. The Lord will respond by destroying the beast and his armies (Revelation 12:7–17; 19:11—20:10). Zechariah 14:2 depicts all nations aligned against Jerusalem at the end of the tribulation period, but the Lord will fight against those nations and prevail (Zechariah 14:3). Then, Jesus will become king over all the earth (Zechariah 14:9).