A drop in price for a game, which has been released two weeks ago, is normal nowadays. Especially for the UK market. I guess Skyrim and Rage failed too, because you can buy the game for 29 Euro in the UK (they are even discounted before release!).
Only Square Enix and Obsidian can answer the question, if DS3 has been a commercial success for the publisher. A much better indicator for success will be DLC and patches.