[Radek]

Aha! Okay. First of all, gain the Divine Sigil if you haven't gained it already. Now, go to the bridge leading to the dragon's lair and jump down. A "strong current" will catch you and take you underwater to an uderwater siphon leading down. Do not object and let the current take you down. Now, swim backwards back. You should emerge from the water at a chest behind two big boulders.
The chest contains the third dragonshard. Remove the boulders by "using" Divine Sigil on them. Return to the bridge.
Notes:
1. If you can cast Create Portal and Teleport then you do not need the Divine Sigil. Create Portal on the bridge, go for the dragonshard and Teleport back. Create Portal and Teleport is the simplest and fastest way of gaining the third dragonshard. Use it preferably.
2. You can come to the boulders from the opposite side. Follow the corridor leading from the bronze nonsense at the beginning of the dungeon to its end.