Albert Barnes Commentary Jeremiah 39:3

"that all the princes of the king of Babylon came in, and sat in the middle gate, [to wit], Nergal-sharezer, Samgar-nebo, Sarsechim, Rab-saris, Nergal-sharezer, Rab-mag, with all the rest of the princes of the king of Babylon." — Jeremiah 39:3 (ASV)

These princes were four:

1. Nergal, Sharezer, that is, Nirgal-sar-usur (May Nergal protect the king);
2. Samgar-Nebo (Be gracious, O Nebo);
3. Sarsechim. No explanation is given at present for this name. He was Rab-saris, that is, chief of the eunuchs (see note on 2 Kings 18:17).
4. Another Nergal-sharezer, who was Rab-mag, that is, chief of the Magians. He is known in history as Neriglissar, the son-in-law of Nebuchadnezzar, and probably his vicegerent during Nebuchadnezzar's seven years of madness. Two years after Nebuchadnezzar's death, Neriglissar murdered Evil-Merodach, Nebuchadnezzar's son, and seized the crown. However, after a reign of four years, he was killed in battle against Cyrus, when disputing with him the crown of Media. See note on Daniel 5:1.

The middle gate — Probably that which separated the city of Zion from the lower town.