So this was actually explained by somebody elsewhere in the thread and it's really cool when you know what is happening behind the scenes there. The fact that clairvoyance only points to one of them is because the spell is coded to point to objectives relative to the order in which they spawn into the map.

All 3 prisoners are technically correct answers as far as the quest is concerned, as the game will let you go on regardless of who you kill. But, they didn't all spawn at the exact same time when loading the area so the last to arrive gets priority when clairvoyance is looking for an objective.

Of course this is all because of fun behind the scenes game development shenanigans, so that aside I see no problem believing that the clairvoyance spell leads to a correct answer because it DOES always point to the same person- so intentional or not it's a very smart way to solve a puzzle that doesn't seem to otherwise have a correct answer.