Invader ZIM: what's with all the three-letter names?

So, I’m sitting here watching the latest episode of Invader ZIM. It features a new student, TAK. She adds to the list of three-letter names: ZIM, Dib, Gaz, & GIR. Is there some signifigance to the characters’ names having three letters?

[ZIM voice] Fool! How dare you question the name of your future planetary warlord! Three letters is all your puny earth brain can handle! Someday soon you will quake in fear at the name ZIM! ZIM! ZIM! Hahahahaha! Even now you tremble before my mighty name!

GIR! Stop that! Noooooooo![/ZIM voice]