Rashi: Initially there were three Alpha Bet'im (sets of 22 verses arranged according to the Aleph Beis) - Eichah Yashvah, Eichah Ya'iv, Eichah Yu'am (Perakim 1,2 and 4). He added "Ani ha'Gever" (Perek 3), in which there are three verses beginning with each letter.

Radak citing Eichah Rabah, Pesichah 28 #1: "V'Od Nosaf Aleihem" hints to Eichah Ya'iv, "Devarim" hints to Eichah Yu'am, "Rabim" is "Zechor Hashem" (Perek 5), "ka'Hemah" is "Ani ha'Gever" (Perek 3), which is a triple Aleph Beis ¹ (three verses begin with each letter).

Radak citing Eichah Rabah, Pesichah 28 #2: "V'Od Nosaf Aleihem" is "Ani ha'Gever", which is tripled. This teaches that initially, the Megilah had three (Perakim according to the Aleph Beis) - Eichah Yashvah, Eichah Ya'iv and Eichah Yu'am, and Nosaf Aleihem Devarim ka'Hemah (another three Aleph Bet'im). "Rabim" is "Zechor Hashem."