Welcome to the board, seeshel! Sounds like you're a shore-dweller like me (unless, of course, you just liked the name). I think you'll find this a stimulating home for your linguistic inquisitiveness!

Trilithon means three huge pillars of stone with a heavy slab across the top.

I'm sure that's true, Dr. Bill. But I'm wondering, if one stone pillar or statue is called a monolith, then why isn't an arrangement of three called a triolith instead of a trilithon?