The dwelling places and assemblies of Mount Zion mentioned in Isaiah 4:5 mean, respectively, the good of the celestial church and the truths of that good.

(References: Apocalypse Explained 594)

The number three in the Bible represents completeness. That's why Jesus rose on the third day after the Crucifixion -- his transformation was complete. It's why Elijah stretched himself on the dead child three times, why Jonah was in the belly of the fish for three days, why the Lord called Samuel three times, and many more. It's also reflected in the fact that there are three levels of heaven, three levels of hell, three levels to human life and, ultimately, three aspects of the Lord Himself: Father, Son and Holy Spirit.