This Runebound Coffer is quite tricky to open, but I managed it so now I am here to help you do it too.
The First rune I found was the one at ~70,31 - in front of the altar in the corner of the courtyard. (East/northeast of the chest)
The second rune was tricky to get to alive, it is inside the Hall of Elixirs at ~71,34. (South of the chest)
The final one was the hardest, mainly because it is invisible. It is due east of the chest, at ~71,32 - in front of Moret the Vogue, under the carpet in front of that Lich.
I hope this helps you out in some way.