The Indiana Jones riddle refers to a scene in the Indiana Jones movie "Raiders of the Lost Ark", in which Indiana Jones, played by Harrison Ford, needs to cross a floor of tiles. The only correct way to do this is by walking on specific tiles only. This was implemented in Baldur's Gate 2 as well (possibly as an ode to Indiana Jones from the Bioware-team).
The shadow you killed (near the stones with the letters on it) is the part corresponding with the Indiana Jones riddle.
SPOILER
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Location of all three parts:
- Give the Bones of Amauna to her ghost in order to receive the "Dawn's Light" piece of the symbol.
- Answer the statue's questions correctly to receive the "Sun Ray" piece of the symbol.
- Defeat the shadow near the lettered floor to get the "Lightstone" piece of the symbol.
All in all, you should follow the hints for third piece to get on with the game. Search for the shadow's remains and you'll find the piece. If you have the BG2 expansion pack Throne of Bhaal installed, then just hit the TAB-key to locate the body more quickly. Otherwise you're forced to mouse over the screen, unless, of course, your photographic memory helps you out on the location of the shadow's corpse.