Wow, lots of questions to answer. I will get back to them in a couple of hours, now I have to run, but for now let me just say that I have spoken to the former DM again about the puzzle and since the campaign is over, he gave me several powerful clues that I can share with you:
1) Algolei's answer of filling in the letters into the blanks is wrong.
2) The blanks are to be filled with only the three symbols that are present in rest of the grid.
3) The symbols have no significance outside of the puzzle and there is no significance to them being triangles or circles or arrows - you could replace them with any other three symbols and the puzzle would be unaffected.