The prophet is commanded to perform a dramatic, symbolic act using his own shaved hair, illustrating the total devastation awaiting the inhabitants of Jerusalem. The very act of passing a razor over his head represents complete destruction and annihilation [רד"ק]. The hair is then divided into three equal portions, reflecting the distinct and tragic fates destined for the people.
The prophet is instructed to take the first portion of hair and burn it with a small flame that gradually grows into a consuming blaze [מלבי"ם, מצודת ציון]. This burning takes place directly upon the brick that he had previously engraved with a map of Jerusalem, timed to coincide with the end of his symbolic 430-day siege [רד"ק, מצודת דוד, מצודת ציון, ביאור שטיינזלץ]. The primary approach among commentators is that this act represents the residents who will perish inside the city walls from starvation and plague as the siege reaches its end. The severe, agonizing hunger they will experience is compared to a fire burning the body from within [רש"י].
The second portion of hair is to be struck with a sword all around the perimeter of the engraved brick [רש"י, ביאור שטיינזלץ]. This signifies the people who will be killed by the enemy's sword just outside the city walls during the conflict [מלבי"ם, מצודת דוד]. More specifically, it points to those who attempt to escape Jerusalem, such as King Zedekiah and his followers. They were pursued, overtaken, and killed by enemy forces in the plains of Jericho, in the area surrounding the city [רש"י, רד"ק].
Finally, the last portion of hair is scattered to the wind, blown away into the air [מצודת ציון, ביאור שטיינזלץ]. This illustrates the fate of the exiles who will flee and scatter to various corners of the earth [מלבי"ם, מצודת דוד]. However, they will find no peace in their new locations. God declares that He will draw His sword from its sheath to pursue them [מצודת ציון, רד"ק, ביאור שטיינזלץ]. The primary approach among commentators is that even in their places of refuge, God will provoke enemies to shed their blood [מלבי"ם, מצודת דוד]. Some commentators identify this specific group as the exiles who fled to Egypt alongside Johanan the son of Kareah. Despite being warned by the prophet about the sword, it ultimately caught up with them when Nebuchadnezzar invaded Egypt and wiped them out [רש"י, רד"ק].